Question 1: Will Epic stop buying exclusives and, if not, to what extent will they keep doing it?
I think Epic will keep on pumping only as long as they need to. It is clearly a loss-leading strategy which means that as soon as it's not necessary anymore, the miracle of free moneyhats will go away. For good, I should hope. The extent is impossible to predict without knowing their finances nor what the board of directors and decision makers have in mind for their global PC gaming strategy.
Question 2: Will Steam decide to lower its percentage and, if so, what will happen afterwards?
No. Absolutely no reason to at this point in time. It isn't the industry standard, nor is it viable to match anything approaching Epic. A discussion about percentage also is meaningless without adding to it context. Regional pricing, and how much the platform absorbs. Not to mention that the percentage never was the point considering that Steam's percentage can already be 0% through key resellers. The only thing that could push Steam to lower its percentage would be a global market shift across all of gaming. In that light, they are already lower than average with the new cut scaling.
Question 3: Will Epic improve its service and, if so, what will the reaction be?
If EGS's short history is anything to go by, no. They will taunt people on Twitter, further damage their image toward core PC gaming and stir up controversies. I have no doubt that within such a large amount of time, some work will be done but I don't trust Epic to put any significant resources toward a better customer experience. The customer is, ironically, not their focus. I surmise that the reaction will be articles praising such "disrupting" and "daring" choices, comparing them to charter airlines and other such spin ignoring the reality customers and the PC ecosystem might face as a consequence.
Question 4: Will Epic even be around in the long-term?
I do not think they will. The EGS has no relevancy or reason to exist beyond removing consumer choice. Even if they somehow came to their senses and focused on service, not anti-competitive behavior, they have scorched too much earth with PC gamers. I don't give them long once Fortnite - or investors - forces them to reevaluate the financial viability of their store as well as ongoing development efforts.