That's an interesting question. We don't have any current plans along those lines right now, but it's something that we can look at, providing some way of either changing your secondary stats or getting a partial refund. The thing we'd want to be careful to avoid is a world where it feels too easy to make one legendary for one situation (PvE), then disenchant or change it to a different one to go PvP, and you don't really feel like you ever need to build a collection. But I agree that making a mistake there feels like it could be frustrating, and we should do more.
Yes. Chains of Domination will definitely add a lot of catchup mechanisms, both for alts, but also for returning players, for new players who are joining Shadowlands and hitting level 60 for the first time. It's going to be much more important for you to be able to get into the new content, to get a legendary without having to wait weeks for it, so that you can play with your friends and experience what everyone else is doing. So that will benefit both new and returning players, and alts.