I land midfoot/forefoot but i still need a bit of support i think. Its possible that you can still overpronate in the forefoot cycle of the gait. That may or may not be an issue but i have found i have had issues when i have gone down to less supportive shoes.
The sizing guide for the 2xu version suggests that if you are on the border you should go for the smaller size. I was the same and went down to medium rather than large. They are snug but nor painfully tight. I dont think you want them to be restrictive but you definitely dont went them slipping down.
Have you dont a 10k recently? If you have there are many pace calculators what will give you an idea of equivalent times. Otherwise it is hard to say what you should do. HM pace is just on the hard side of easy. It ought to feel fairly comfortable but you should still feel like you are pushing yourself a bit. Of course that needs to be sustainable too
To be honest you are better to run to pace rather than HR in a race. You are going to ahve adrenaline on race day causing changes in your rate and while you might average 85% you will almost certainly get some drift so there is no point starting off at that pace. Once you have done a few with the monitor on you will have a better idea of what your HR does and you might then consider using it in the race.