Thank you! Great to hear it's working out well for you there. We wanted to create an art program which was affordable and easy to get into rather than a whole ton of complexity. We're always grateful for people spreading the word!
In answer to your questions:
1/ ArtRage layers are compatible with Photoshop's layer system. You should find that Elements 4 will be compatible with this. Our support is basically standard raster layers with blend modes.
2/ ArtRage doesn't support TIFF currently. We do support JPG, PSD, PNG and BMP outside our own format, PTG ( painting files )
3/ Oh, not sure on this one I'm afraid! I think we have a number of web developers on the forums though so someone else may have some ideas.