Sorta, I'm a complete Lore Fool, so please be gentle.
With Project Purity activated at the end of Fallout 3 as well as the activation of Liberty Prime (both achieved without the Outcasts)
Lyons has demonstrated that protection of the population of the wastelands (in particular the intervention of the Lone Wanderer) have validated his decisions. he has a massive robot, and fresh water.
He has achieved the BoS long term goal of improving the world via technology, and after the events of Broken Steel the Enclave may be weakened. He is at present in a very stong position.
Conversly Casdin, i still begging for Power Armour etc, and doesn't appear to have a long term plan beyond that.
he appears to be in weakened Position.
1) Would Outcasts return to the Citadel and leave Casdin?
2) If so would the assination of Casdin allow for all/most of the Outcasts to b re-intergrated tothe citadel?
3) with the Enclave gone and if there was not reconcilliation would the Outcasts be numbered as a threat tothe Citadel and thus hunted by Lyons BoS?
It was never the BoS' goal to "improve the world via technology", they claim to be 'saving humanity' but that in itself can be misconcieved. By saving humanity they seek and secure pre-war tech, and 'protect' humanity by making sure it doesn't repeat its past mistakes of destroying itself. They believe there are the few people in the wasteland that are able to use technology responsibly, and in so many words, I agree. The BoS at least, seem responsible enough.
Casdin is indeed in a weakened position, him and his division have been cut off from both Lyons and command back on the West Coast. They can do nothing but achieve their goal to the best of their abilities.
Another question would be, would some of Lyons' division break off and join the Outcasts? So far there hasn't been much of a power struggle between the two, so I doubt the members of the respective factions are questioning their loyalties.
The assassination of Casdin may be a vital blow to Outcast morale, but if they are certain that Lyons' isn't in league with their goals, then I believe they'd continue to be Outcast, you also have to consider if Lyons would show leniance, and let them join again. He may have a soft spot for the local populace, but he is still a strong leader, and allowing them back after such a 'betrayal' would perhaps reveal yet another compromising weakness in his judgement.
The Outcasts wouldn't be a threat unless they made it so. If the Outcasts decided to fight Lyons, or interfere in their efforts, then the two might meet at the end of eachother's gun barrels alot more often.