Ok, here we go. This will be long. Some of what you're asking is not always easy to do, and you'll figure that out when you are trying it. First I want to be sure you understand what's happening with Trace Bitmap.
In your first message, you mentioned being aware of making strokes wider or narrower using Fill and Stroke > Stroke Style tab > Width. So you know a little bit about paths. Paths are the most basic element of vector graphics (in my understanding anyway). Paths alone (as phiscribe was explaining) aren't really visible. They can exist with a stroke or without a stroke, and the stroke is what you're making thinner or thicker in Fill and Stroke dialog.
The Brightness Cutoff option that you've been using (in Trace Bitmap), as I've explained, gives you an inner path and an outer path. The paths don't have a stroke, but because the paths are both closed and combined, they have a fill. And because the paths are so close together, and filled in between, it looks like a line, to us. But to Inkscape it's not a line, it's a fill. You can see that by selecting one of your traced images with the Node tool. Go ahead and try it, so you can see.
When I look at your butterfly image, I see the image has tons and tons of nodes. Possibly you don't need so many. And for editing, it's probably better if you don't have so many. Fortunately, there's an easy way to fix it. Path menu > Simplify will reduce the number of nodes drastically.
Unfortunately, and it's kind of a long story why, but in your drawings it results in some distortion of the "lines". Go ahead and try it. If you use the butterfly drawing that you shared earlier, you'll see what I mean. It's possible to adjust how many nodes are removed with Simplify, but I'm not sure if that would fix this problem. Let's see....
Oh, it did help quite a bit. I still see some distortion. But that result would be acceptable for me. But of course it's entirely up to you. Ok, go to File menu > Inkscape Preferences > Behavior > Simplification Threshold, and change it to 0.0010. Then Undo the first simplify, and try it again with the new setting. If that result is ok for you, it will make it much easier to edit the paths.
To edit the paths, with the Node tool, you can grab the nodes and move them around, or you can move the handles around, if you don't want or need to move the node. Grab that tiny circle on the end of the handle and move it. Or, if you place the mouse over the path (where there aren't any handles in the way), you can drag the path itself. So this is the best way to edit just a few areas of a drawing.
If the simplification results are not acceptable, and the distortion it creates is too much for you, you don't have to do it. As long as you only have one or 2 drawings in a file, the large number of nodes won't cause any problems. It will be harder to edit the drawing with so many nodes, but you won't have any performance issues. But if you start putting a lot of these drawings, or more complex drawings in a file, you could start seeing Inkscape going slower and slower. But for all the info I have at this time, it sounds like simplification is not a requirement, just an option.
Ok, so you can see that node editing can be a very tedious process. You certainly wouldn't want to edit a whole drawing that way. If you need to make a whole drawing thinner, the best way to do that is when you make the trace, rather than afterwards with the Node tool. Notice the Threshold spinbox (where you can change the number value) to the right of Brightness Cutoff option. You can adjust that and make the result thicker or thinner. It doesn't give you unlimited ability. For example, I don't think you could make the line as much as double its width, or half it's width. Well, maybe half, depending on how wide it is in the first place. This is the best way to make the whole drawing thinner.
To make the lines of a whole drawing thicker, if you want them thicker than you can get using the Threshold setting, you can add a stroke. Select the new paths. Then hold the Shift key, while you click on the black chip in the palette. If that made it too thick, you can go to Fill and Stroke dialog, which you already know how to do, and adjust it from there. Remember to have the zoom set for 100%, so you're judging the thickness properly.
Regarding the centerline trace extension, what is it that you don't understand - how to use it or how to install it? Unfortunately, the author of that extension doesn't seem to know how to write instructions for people who aren't tech-savvy. But I think I can decipher them. I'm less sure if I can deciphere how to use the extension. When I click OK, it seems to work, but everything else is still a mystery. If it's help installing it that you need, tell me what your operating system is, so I know which section to decipher.
If you're ok with the centerline trace results, that's probably your best option, compared to everything above.
Since you expressed wanting to learn how to do things, I decided to go ahead and tell you all that. But what I'm really thinking is that your best option will be to recreate your drawings in Inkscape, using traditional path tools, rather than auto-tracing them. Or in other words, manually trace them. Although if you like the centerline trace results, that will be faster. But I still think this would be the best option. Or you might find that one of these techniques works best for certain drawings or other techniques for others?
To manually trace them, you still import them as with auto-tracing. You might like to put the imported drawing on its own layer, which you can lock, which prevents you from accidentally moving it. Sometimes it's hard to get it back in the right place, once it's moved. Layer menu > Layers, opens the Layers dialog. Click the plus sign to add a new layer. In the little dialog that comes up, select Below Current, in the Position dropdown menu.
Import the drawing. It should import onto the new layer, but if it doesn't, you can move it using Shift + PgUp or PgDn. When it's in the right layer, click the tiny lock icon, to lock it. Now you won't be able to select it, so be sure you have it in the right place before you lock it.
Now enable the layer above it (by clicking on it, in the Layers dialog). Enable the Pen tool, and you're ready to start drawing. I think I'll just make a little video for this. It will be faster than writing it all out.
I should note that I use the Pen tool a little differently than most people. In Regular Bezier mode, you can make both corner nodes and smooth nodes, as you draw. When you click once with the Pen tool, you set a corner/cusp node. When you click-drag, it makes a smooth node. Some people can create the nodes, and adjust them perfectly, on the fly, so to speak. I can't seem to coordinate my hand/finger/mouse to do that. So instead, I put the Pen tool in Straight Line mode, which only sets corner/cusp nodes, and won't let you create a smooth node, so it only draws staight lines. Then I come back later and change the nodes, all at once. I'll try to indicate where these modes are, in the video, I'll point to them with my mouse.
A couple of things in the video that aren't obvious. The video software puts a little blue circle around my mouse, when I click. So you can see whenever I click. You have to double-click with the Pen tool, to end a path, although the video software does not indicate that. It's hard to see the new paths, because they are black, and only 1 px wide by default. So you can see what I do, to make then wider, and I made them blue, so you could see them. To change the color of a stroke, hold the Shift key while you click on a color in the palette. (To change the fill, just click on the color, without any keys. Although I guess you won't be using the fill, for this.)
So once you have all the paths made, you can select all (Ctrl + a) and change the width in Fill and Stroke dialog, all at once. Once you become familiar with the Pen, you can finish tracing the butterfly image fairly quickly. If you have better coordination than me, you might be able to set the smooth nodes on the fly, and go even faster.
Just in case, I thought I'd mention some little hardware that you can connect up to your computer and Inkscape, called a graphics tablet. That's a different kind of tablet than iPads. You can draw on them, with a special pen. So you could make your drawings directly into Inkscape, and avoid all this tracing! I have no idea how expensive they are. I think relatively reasonable. (If I had any sketching skills, I would definitely buy one! But that's just me.)
Regarding the coloring books, I don't see why some religious drawings wouldn't be ok. Depending on how "big" this project gets, it might be possible to have different themed books. Some for adults, some for children, and maybe you could even make one for your church. Although I should say that I have no idea how much this might cost or how much the books would have to sell for, to cover the cost and still have a little donation for Inkscape. And beyond that, I'll have to figure out how to sell them.
Anyway, I removed your email address from your message, just for safety. But could you send it to me privately? Click on the My Messages tab in the main menu, and that will open Private Messages interface. That way, spammers can't find your email address. Let me know if you need help with that.
Ok, I think I covered everything. But be sure and ask if you have any questions.
