can i use the regeneration in halftime to get 5% condition on a player if i subitute him pre-half an substitue him again in half/ immidiatly after halftime?
You can't do this.
The subs have more to do with the performance/presence of a player in the game than the condition.
For condition you must have the team ready before the match started.
Also subs are for changing / counter your formation.
Search for players that you don't read their names so often or they have some fails in the comments.
I almost always sub a player that gets a yellow card in the fisrt half.

I read in the forum some other comments like :
don't sub a DC if your team is winning 1-0.
If you make a sub round 60-70' min you can have an extra goal from sudden but I can't tell if it's a fact.