I personally would focus on mp over hp if you don't need the hp for another job (I have more hp merits because of ninja and samurai, but if blm is your main I'd stick with mp). The mp is going to be vastly more useful for you.
The stats thing may be a bit harder if you are worried about multiple jobs, but I'd say the main things to consider there are 1) which job will you be using more often, and 2) how easy is it to get that stat in other places (gear, food, atma, etc). It seems to me to be pretty easy to hit stat caps in Abyssea with atma, so I would focus on other merits for now, but again, that depends on which atmas you have and how much you'll be doing outside Abyssea. If you can't decide, then you can always half and half it, but I have no other suggestions on which one to pick, so maybe somebody else can give you better advice.
According to SE's roadmap
for this year, the cap increase to 95 will be happening in August. You will want to stack as much ice potency as you can even afterwards, because Atma of the Beyond makes ice our best spell in Abyssea, not to mention the fact that ice is just an all-around good element due to fewer resistances and numerous weaknesses. As for fire vs thunder, all I can say is that we have a higher tier fire spell than we do for thunder currently, but other than that I don't know there are really benefits of one over the other. If you don't mind redistributing merits later, then feel free to go for fire. I personally just keep thunder potency for use outside of Abyssea, since I very rarely cast anything other than ice while inside.
As always, I don't claim my opinions to be in the majority, but I only give what I feel to be good advice. If somebody more knowledgeable than I wants to correct me, feel free.