(1) I'm not sure I fully understand what you are referring to wrt the "acks" but either way, one major difference is that the acks don't have to be made durable in the alg. described by the post. Also, the other key thing to note is that alg. described by the post does not suffer from the blocking or cloggage problems of 2PC.

(2) The post was already too long, so I didn't cover all the cases. The code rewrite algorithm described in the post is indeed O(n^2) if every shard has the possibility of a state-based abort. However, the number of shards that have possibilities of state-based aborts is known before the transaction begins, and since the same values are being sent everywhere, you can always use standard network broadcast techniques to bring the complexity back to O(n).

(3) Garbage collection can work via a high water mark. You keep track of the highest transaction number for which all transactions less than this number have been completed, and can garbage collect values for those versions.