Ah... How convenient for him. He not only got the transfer he wanted, he also had a nice vacation too. Lol. Was he paid the whole time he wasn't doing anything? He should be training at least, to keep himself fit for his return to the games.
And as soon as the announcement was made, Costa went back to spain and started working with Atletico Madrid fitness coach Oscar 'The Professor' Ortega, so yeah, he's been training for the last 3 months so I'd say he's fit to play.
To bring Eden Hazard to RM one player has to go, and I think that player is Benzema. But we'll see what happens with Bale, if he keeps getting injured then, the obvious choice would be him.
And I don't think Zidane will be sacked if PSG beats Real Madrid, Zidane has done what no other football manager has ever done, to win 2 champions leagues in a row, also, real madrid won 5 trophies last year (first time for real madrid). With all that in mind, I think the only reason why Zidane would leave RM is because he made the choice to leave the club.