some questions about mastery
1. I don’t think so. Definitely not HoT Masteries, of course.
2. I don’t think so.
3. No. Just the opposite; you will be ahead of the game, so to speak.
Good luck.
1 & 2) No, XP you gain before unlocking masteries won’t count towards them.
I know this because I pre-purchased HoT but when it came out I was too busy with Halloween so it was about a week after release that I actually started playing it. When I unlocked masteries I had no progress on any of them – the HoT ones or the Pact ones.
3) Hero Points have nothing to do with masteries. Their only use is to unlock skills and specialisations (traits).
I’m not sure what you mean by putting yourself at a deficit. Getting to level 80 will get you enough points to fully unlock all the base skills and specialisations, and completing hero challenges will give you extra ones on top of that, which can then be used for the elite specialisation. According to the wiki there’s a maximum of 1012 hero points obtainable, and it only takes 648 to unlock all your skills and specialisations, including the elite.
So the “worst” thing that could happen is you unlock everything and still have points left over. If you’re short of points to unlock everything it just means you need to do more hero challenges, not that you’ve done ones you shouldn’t have.
“Life’s a journey, not a destination.”