I am pretty sure that they only gain the 5% After the match, because Everytime that hapened to me, my 3hours to the next gain, only started to countdown After the match end.

About losing possession with players out of position i only seen it happen After having 4 players out of position.
I normally play 1 or 2 players out of position without any problem.
About they performing worst, thats bull**** i have a ML/AML that in yesterday game played as MC scored 3 goals and was MOM, last season he played almost all season as MC and always got high ratings, was one of my Best scorers and assisters.
I also had a DMC that i had to play him as DC and was 3 times MOM.
I have a triple role player that playing ST is wonderfull but as AMC he plays awful.
It depends of the player itself, you only know If you try, but normally If you put him near is natural position you will not have any problem, i even already played ML as MR with good results.