Source code is rarely released for commercial games (but sometimes leaked). River Raid included. The source we have for 2600 River Raid was decompiled (reverse engineered) by Thomas Jentzsch, and later I added to his work on it. Someone could do the same with any game, including the 800 version, but no one has as far as I know. What Jentzsch did was rare.
Yes, the 800 version was different enough to be at least a version 2 if not a sequel edition. The 7800 could likely run a version that looks a lot like it. I just didn't go there because my purpose was entirely personal to relive the 2600 original version, just in 2-player (and with prettier sprites just because). I've never played the 800 version or any version but the 2600 one. The 800 version looks like it uses the same exact level generator, just with jagged borders and new enemies replacing old ones. Thus, it probably would not be hard to match up the same level code in the binaries and then decompile the changes surrounding it. But, that's not on my todo list since this was just a personal side project.