GIMP Version: 2.8.14
Operating System: Windows
OS Version: windows 7
GIMP Experience: New User

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I have several (15) tiles of a 72 inch drawing that I am trying to turn into one large digital file. They are black and white and they have to be nudged into place with the adjoining tile and then "tweened" - stray pixels cleaned up because the hand drawing doesn't quite match perfectly and I don't want there to be a visible line.

I opened a .tif with one drawing, and opened another .tif with the other drawing. I changed the canvas size of the first tif to double the height, set transparency in both .tifs to white, and copy and pasted the second tif over the first tif. I lined it up until it matched as close as possible, zoomed in, and selected the pencil tool. I could not make a mark with the pencil tool. No matter what or where I clicked, no visible mark anywhere. The heal tool had the same effect. What I am doing wrong? Trying to draw on the wrong layer? Wrong file format? Something wrong with the transparent color?

maybe you missed to set each layer to image size?

Have you tried saving the image as a native XCF file, than reopening the image for editing?

maybe it's not related but sometimes i have my brush/pencil tool options set to clipboard instead of some other brush, and if i don't have anything in the clipboard then it draws nothing as well.

Can't help with the rest of your question but re painting and healing have you got more than one selection? (Has tripped a few of us up) if you have a selection apart from where you need it to be.