Would Gallas and Pappenheim succeed in leading the imperials to victory (or at least avoid losing more than OTL)?To answer your questions:
Who would take over? Well, after Tilly is dead, probably Gallas (as in OTL) and, after he is compromised by a lousy performance, Ottavio Piccolomini who ended up being the last Hapsburg generalissimo of the 30YW. Actually, even Gallas was seemingly capable but his army suffered from a lack of a discipline (inherited from Wallenstein) . Or, based upon the rank, Pappenheim who already was a field-marshal, quite popular and personally favored by the emperor (he got the Golden Fleece, which Wallenstein never did).
W coming to GA service is not a fast winning scenario you are afraid of. Of course, Wallenstein is a very respected figure (during the siege of Prague the Swedes took care not to damage his palace) and immediate benefits are seemingly obvious. But he is also a primadonna and handling of the primadonnas is a tricky business for which GA (unlike Ike) was ill-suited being himself a great general. So having such a subordinate commanding his own army and having his own political ambitions could easily turn into a very explosive affair. Not to mention that their warfare styles had been substantially different, which is a ground for potential problems on its own and that GA is never going to trust W completely (unlike a truly subordinate figure like Bernard of Weimar).
Now, if we assume that W is given the Mecklenburg duchies and promised Bohemia, there are two options:
1st, “to promise is not to same as to marry” and GA may be either incapable or unwilling to deliver. Result: Wallenstein is pissed off and tries to reapproach the Hapsburgs. If situation is lousy enough, he can be taken back with a variety of the follow ups.
2nd, Bohemia is getting a new king and King Albrecht is not interested in a further destruction of his country and tries to arrange for peace. Result: GA is pissed off while Ferdinand is unwilling to accept the loss and Wallenstein is trying to get guarantees from all possible corners that his rule lasts after the peace is concluded. The most obvious ally is France.
Then there could be scenario in which W joins GA but then comes a higher bidder, Richelieu, who sees that GA keeps taking the French money but pursues his own goals. The French national army is not quite there so Richelieu is buying Wallenstein with his army (pretty much as he bought the troops of Bernard of Weimar but with a greater success). Of course, W is going to lose his HRE possessions but France has means to compensate him for the losses both in France and within the HRE (during the peace negotiations).
Of course, it is necessary to keep in mind that we are talking about both branches of the House of Hapsburg and that the Austrian branch heavily relied both on financial and military help of the Spanish branch (hence the “infantry cardinal” ) and that for both the Spanish branch and for France the main theater had been the Netherlands with the operations on the Rhine being mostly a way to protect the route from Italy to the Netherlands (for Spain) or to close that route (for France). “Super-efficient” Swedes still would have their main interests elsewhere (actually, it does not look like anybody, including his own Chancellor, could tell for sure what exactly GA was trying to achieve in Germany) meaning that the war would keep going on. And, with the Napoleon not being anywhere around, even “super-efficiency” hardly would result in a decisive victory because the armies were too small and the rulers usually did not give a s—t about their subjects being looted and killed (by their own troops as often as by the enemy). And many of the HRE princes earned quite soon that GA is not really an attractive alternative to the Hapsburgs (as long as the Hapsburgs are not too domineering) so neither side is going to run out of armies too soon. So we are still looking for a war of exhaustion.
"Primadona"? I don't know what that means, can you enlighten me?
How do you see the relations between G2A and Wallenstein if the latter joined it? What were the differences in military strategy between them? Could this lead to a Swedish "defeat" because of dissension in the camp?
If Wallenstein was frustrated with the role he was given, how would the Habsburgs treat him if he returned? By giving him what, by the way?
If Wallenstein succeeds in becoming King of Bohemia, are his chances of remaining so weak or very weak?
If Wallenstein is supported by Richelieu, how will Richelieu continue his funding to Sweden? Moreover, if France wins, what does Wallenstein receive in France and in the HRE for his services?