I doubt that the dpi would have anything to do with this problem. If you want to export a PNG at 90 dpi, you would not have to install an earlier version. Just change the dpi in the Export PNG dialog.
I'm not familiar with the extension you mentioned, Prepare for Export. But you could look and see if it allows you to change the dpi yourself.
If it doesn't, then to answer your question....oh gosh, I can't remember for sure. I'm pretty sure version 0.91 was the last which used 90 dpi as the default. If it wasn't 0.91, then it was 0.48.5 (yes, the project jumped ahead in version numbering).
Note that your image might not be the right size, using 90 dpi (in case size is important for your project).
However, I think saving as SIF is currently the proper way to get images from Inkscape to Synfig. It's possible those older extensions don't work, although I don't know for sure.
I don't know what you mean about the "extra edge" that someone's text has, which exported properly. Do you mean it's a serif font?
Have you tried different fonts? It doesn't sound like it would matter....unless it was some kind of font which doesn't have closed spaces.
What about different file formats? Are there any other file formats which you could use, to get the text from Inkscape into Synfig?
I think I would probably go ahead and file a bug report, at this point. Although I realize you're probably more focused on getting the text into Synfig correctly. But once developers can have a look at the problem, they might be able to offer a workaround. Or maybe they might even know a fix?
OH! I just had a flash. Look in Inkscape Preferences > Input/Output > SVG Output > Path Data. How is that set? For general use, Optimized is the best choice. But maybe for your purpose, Absolute would be the best choice? Change that before you type the text, then convert to path, then Union, then save as SIF.
If that, as well as using a 2nd layer doesn't help, then I'm fresh out of ideas.