1) Yes, they can. If they couldn't my DirectShow stack would need to be a lot more complicated. I have decoded XviD with DivX and vice-versa without hassle. Maybe this is to do with profiles or something? I am happy to believe you could encode something with one that doesn't decode with the other, but in theory and in practice it seems to work out in most cases.
2) My worry is how different these can be without causing problems down the line. VP8 as a specification seems comparable to early days HTML, whereas MPEG-4 part 2 or part 10 (h.264) is more like HTML4.
1) In the end, both xvid and divx (and other, like ffmpeg or nero) decoders contain fixes for divx/xvid quirks. In software, it is easy. For extra fun, get your hands on some dvd players, that claim divx support. Yes, they can decode divx, but decoding xvid is much bigger problems for them (it is not about profiles, but about bugs in produced bitstreams).
2) My worry is how different these can be without causing problems down the line. VP8 as a specification seems comparable to early days HTML, whereas MPEG-4 part 2 or part 10 (h.264) is more like HTML4.