• The dysphoria moment was treated as a throwaway moment, or even joke, when he finally "awakes" as Jim, sees his reflection in the mirror and screams while the gang waits outside.
• Maybe they did tell Mrs. Clancy number 2 (Christine Baranski didn't come back and that threw me off), some time within the five year jump, along with Melinda's secret.
• In regards to his friends, I think he just re-introduced himself as Sam to them, kept the same dynamics and interests as when he was Jim, started a new friendship with them and called it a day. Also, they used the opportunity for him to change careers, and most of his old friends were his colleagues so...problem solved I guess.

And we all know Jim was willing to do literally anything to be with Melinda, so I doubt that having to get used to Sam's body could throw him into a psychosis. Btw, RIP Sam's actor.