Regarding the group of Ultramarines, I would presume they aren't the grimderp versions that we've seen in some of the dumber bits of canon fluff. At the very least this particular group are certified badasses - for 900 years they've survived the perils of a hostile galaxy with no apparent port of call they could return to for resupply, repair, and reinforcements on a regular basis. Instead they've had to move from place to place, recruiting as they went, keeping their ship intact without a proper shipyard to maintain it. The fact that there's even any survivors at all is miraculous.
The fact that they've survived this long also indicates the group is adaptable enough to not take the Codex Astartes as an absolute set of rules, because the Codex Astartes likely does not cover the situation of "what to do if the Imperium falls". And unless they've got some wrecked Dreadnoughts we can reverse engineer (which they might, given they had the 2nd Company), they'll need to change their organizational doctrine anyways.
And a big benefit of them being Ultramarines is that they have really stable gene-seed, with no known flaws that we'd have to deal with.
Now, as far as their negotiating position goes, it's not weak but it's not strong either. They have something the Trust wants, that being the ability to make new Space Marines, but they are stuck with us until the Warp Storm dies down and we have nearly everything they need to build their forces back up. They also will probably cry tears of joy when Saint Lin tells them that the Imperial Trust was given a mission by the Emperor himself to survive until he's reborn, so the chances of them wanting to leave are slim. That said, they do have options in regards to which worlds they could recruit from, which does give them some leverage with regards to where they end up staying.
I'd sort of prefer that. We'd get control of the gene seed.
The Black Templars have more than likely fallen to the Abomination given their Chapter's culture, so their gene-seed would be corrupted and useless. If they show up, it will be with a crusade fleet to try to purge us as heretics.
Depends.
Its a split between abomination or Khorne really.
For all that the BT were hateful bastards they were non traditional as hell.
They were Emperor fanatics, though, so the Abomination is much more likely. The notion of his death would be considered heresy, and they'd likely readily accept the Star Father as being the ascended Emperor rather than go over to a Chaos God whose forces they'll have fought against and already been aware of.
30 Space Marines are like a joke compared to any military units we could use.
Remember the Sons of War?
30 Astartes can be made into thousands, given time. They are elite units and can be used for appropriate actions, and will have their own hero units.