The question I have is if Philly would agree with you on 2's value versus 11. If so, the Spurs might be able to cut GS out and trade Murray, Gay and Aldridge along with 11 and a lightly protected pick for Simmons and Horford. Then do a DeRozan trade like the ones Dallas fans keep talking about on RealGM (DMDR for THJ 18 and filler). Maybe you could just throw in 18 to remove any future obligation. Or you could use it on a player to fill in the thin guard rotation (Lewis, for example).
I agree with you on standing pat, though. I don't see why that deal would need to be done on draft night. Simmons has too many problems to rush a deal, especially since the team wouldn't really be a contender after the trade.
I also don't think Simmons is a full-time center. But he could definitely play one in a pinch.
I don't think the Embiid trade is going to be a reduced price. He'll go to someone before he goes for a package where the Spurs could save some money. They're saying Joel may well be dealt this off-season. If that's what the team wants, they'll do it, likely before draft night.