Welcome to the forum!
I'm not exactly an expert with dpi issues. But this is the simple version. In Inkscape, when you export the PNG, and you change the DPI, it changes the size of the image. But it also changes the DPI. When it's time to print, then you set the size of the image to whatever you want, and the DPI remains the same. No matter what size you print, it will have the DPI that you set.
In the case where you're uploading it to a website, either you write in the html code the size you want for the image, or if it's a site like amazon or facebook, they probably already have the image size fixed. So that when you upload it, it will fit in the frame which is provided, and it will still retain whatever DPI you set. I would suggest to try uploading it, and see what happens.
Using GIMP for comparison is not a good idea, since I think GIMP uses a different native resolution. So it will always give a different size.
I'm not familiar with the program you mention "Canva".....or is it a website? In any case, you can find out about transparency with Inkscape. The Inkscape background is transparent by default. In Document Properties, you can enable a checkerboard background, if you like. But the background is transparent whether you use the checkerboard or not -- Unless you change it on purpose.
However, if the place where you previewed it normally does show the checkboard to indicate transparency, and it doesn't, then it sounds like you might not have ended up with a transparent background. Isn't there some kind of simple image viewer on Macs, where you can open the PNG and check the background?
If you could share the SVG file, we can help you sort out the opacity problem. It's hard to give suggestions without knowing anything about the image, what it's made of and how.
