Fenix was thought killed on Aiur - however, Protoss Dragoon technology was specifically designed for just his such situation. Either the Dragoon technology was able to actually resurrect the recently-dead, or he was not *quite* dead, but certainly very close.
Fenix's death on Korhal, however, had nothing to do with Raynor's "obsession" with Kerrigan. After his encounter with her on Char, he had stayed the hell away from her, and considered her simply lost. Gone. The Sarah Kerrigan he knew was dead, as far as he knew. Kerrigan came to him and Fenix on Aiur to initiate their shaky alliance against the UED, sparking events from the end of the UED's invasion of Korhal onward. From there, Raynor and Fenix *both* agreed to work with her toward the goal of ending the UED and the new Overmind's threat. It was Fenix's forces that she had to fight through to reach his Dragoon, not the Raiders. While Jim's forces were no doubt present, his forces were not shown in that battle, and as such can be considered to have not been a major presence when Kerrigan ambushed and betrayed the Dominion and Protoss forces, following Korhal's liberation.
Tychus was killed because of *EVERYONE'S obsession with Kerrigan, not just Jim's. It was Arcturus who sent him out in the first place to steer events toward Kerrigan's death. It was Valerian's plan to de-infest the Queen of Blades, and it was Jim Raynor who agreed to work with Valerian to rescue Sarah Kerrigan. Tychus knew that with Arcturus holding the gun to his head, he needed to stay the course. But fighting alongside his old friend again, he began to question himself. His self-preservation warred with his loyalty to his buddy, little Jimmy Raynor. When the moment came, he hesitated. He held the rifle steady, aiming at Kerrigan, and gave Raynor time to make the choice. He didn't want to have to betray his friend... but in the end, it was him, or Kerrigan. As he took the shot, Raynor blocked it, and shot Tychus instead.
Now, I will grant that Stukov gives a precedence for ressurection and infestation all in the same package, but that was during the time when there were still cerebrates around operating their own agendas. Kerrigan has since hunted them all down and eliminated them, and the Swarm is now under her complete control, and she's stopped such experiments with humans. There are no zerg forces *left* to collect his body and infest it/revive it. And even if there were, they would have to pull it off. If resurrecting dead humans through infestation was easy, there would be a lot more than just Stukov on record.
There may be precedence for resurrection via Protoss or Zerg forces... but without any such intervention, the chances of Tychus coming back are somewhere between zip and zilch.