Ethics divergence is either positive or negative.
If it is positive, it will push your pops' ethics further from your empire's. If it is negative, it will push their ethics toward your empire's. This has been demonstrated in another answer here and the Stellaris wiki.
So, if your empire is xenophobic, and these pops are fanatically xenophobic, then your empire is less xenophobic. Therefore, if you want them to become less xenophobic, you want to push their ethics toward yours. This would require a negative drift.
If you can't get negative, then at least less positive might work. It's not clear to me if the divergence applies equally to each pop, or if it might just be an average of a bell curve.
If you can get some of the pops down to xenophobic, then at some point they might end up less xenophobic than your empire. At this point, you would need a positive drift to continue to reduce their xenophobic tendencies and push them towards xenophilia.
I've found no upper limit to the number of entries in my situation log, research or otherwise. I also have no reason to think that there will be a limit.
Furthermore, I've found that you can research many of these simultaneously.
At the start of each game, you will encounter several alien life forms that will produce research entries in your situation log. These all pause the current society technology. According to @Kexlox, the research points during this time are not actually spent on the alien life form research, but are instead saved to be doled out later in a slow release.
So, you wouldn't lose any research points, but you might get your technology a few months later than you would have otherwise, depending on how close it was to completion.
But, I've noticed that, for some reason, you can research all these life form projects simultaneously. So, waiting until you have a handful all ready and doing them at the same time can minimize the disruption to your research, as compared to doing each one as it pops up. I've researched at least six at the same time before.
I would not be at all surprised if that turns out to be a bug, and is fixed in a patch at some point.
Update:
I've been playing after the Unity DLC, and the bug mentioned above appears to have been fixed. There's still no limit to the number of entries in the situation log, and you can start several of them at once, but it seems that only one progresses at a time (per research type).
Best Answer
The ships give a boost of 5% per level of the commanding scientist to the planet that's being assisted, which increases to 10% with the Improved Assist Research technology. The assist research mission also steadily gives the commanding scientist experience, and so is a good way to prepare younger scientists to take over once there aren't any more systems to survey.
I've typically only found this useful with dedicated science worlds, like the homeworld (if you decided to build ~4 labs on it) or any world with primitive civilizations where you built an observatory enclave (because those can often produce massive amounts of social research). Most of the research income of a large empire will come from research stations, which this can't improve.