Some quick replies which might help:

1. Availability can change, and yes reward tickets can be changed as you say.
2. Yes, you have to call to book an open jaw.
3. All very unpredictable. You might get an offer to upgrade in advance, but more you will not. Airport upgrades are down to availability.
4. I think the comments are about relative value for the avios saving, and flexibility of premium voucher over the standard one. It also assumes you have other ways to use the voucher. But it still saves you a lot of avios, even allowing for fees. So if it a good deal for you, then you are the best judge of that.