1. Remember that it is a shared universe regarding the multiple authors thing, so you're going to find inconsistencies no matter what.

I have no idea about the Tyranid/Necron timing thing, as due to retconning I think outright contradictions exist (as you seem to have noticed).

2. Most of the Primarchs have ominous-sounding names, due to the heavy use of Latin/Greek/Persian roots. Sanguinus, for instance, is rather evil-sounding but is a loyalist Primarch.

3. Lehman Russ was never a recognized traitor, and the Space Wolves are a loyalist branch of the Adeptus Astartes.

The Deathwatch are tied to the Ordos Xenos (anti-alien) branch of the Inquisition, and the Grey Knights are tied to the Ordo Mallaeus (anti-daemon) branch.

The Grey Knights and the Space Wolves are not enemies, per se. While there have been times in history wherein various Space Marine chapters have come into conflict with each other (examples include Grey Knights/Space Wolves, Dark Angels/Space Wolves, etc) they are not enemies, and so long as they obey the general creed of the Empire and do not fall to Chaos there are special circumstances that can cause conflict.

4. Re: the chapter-bouncing thing. You're right that in Chaos, you can just do whatever. When it comes to the Deathwatch, however, keep in mind that they do not have a specific geneseed: they are very specifically a group made up of veteran space marines drawn from the classic chapters. It's like a "Navy Seal within the Navy Seals" sort of thing...I guess. Anyway, you'll see guys from many different chapters within the Deathwatch, and they usually serve for discrete periods of time before going back to their chapters. I don't think you'll ever see a Space Wolf become an Ultramarine, or anything like that.