The add-on should be out August or September. I would guess the patch would be out either the same time or a little earlier.
Larian has been good about supporting previous titles, even after publishers for some regions had stopped doing so. With additional content planned for D2:ED, which I assume the publishers would have known about fairly early, I would expect that to continue, even with the extra bureaucracy and restrictions involved with the Xbox version, etc.
Your first encounter on sentinel island you can mindread to go up a level, so the higher level you are the more of an experience bonus that will be. I made it to level 18 in the fjords, so that mindread brought me up to level 19. Even with the lower experience from opponents, I most likely came out ahead compared to going to sentinel island at level 16, jumping to 17 and getting slightly more experience per opponent (the extra experience fighting tough opponents in order to level would compensate somewhat for the lower experience later, as well).
In any case, if you at least make sure you go up a level before taking the teleporter to the island, you will get a full level bonus when you get there.
Do you get more experience for quests at higher levels? It helps to delay solving quests, so you get more experience from combat, but I think the experience rewards are fixed (item rewards do get better at higher levels, though).
It also helps to do any mindreading early, so you pay off the experience debt at a lower level, when monster kills give more experience.
Hang on to items with a Wisdom bonus, and you can get a little extra experience turning in quests. For combat I'm not sure if the Wisdom bonus is a huge benefit, considering at least part of the bonus would be canceled out by leveling faster and reducing the experience for some opponents.
Leave Keara's headquarters and the ministry until the end. If you end up not needing to level, then you will not waste time fighting spawned opponents. If you do need to level, then you can reload and not have much to repeat.
