I wouldn't advise to continue to roll this axe, this is good, from that aim for grief or gotter, its not that rare if you farm duncraig.
As for other questions:
1. relics - hard to tell, there is plenty of buff relics that are useful, hard for me to tell what is absolutely the best. Something like bear morph relic could potentially make most difference per single relic. Or some single target attack oskills, usable with axes, without cooldown.
2. Replacing T4 fanged unique helm, you can get more value from rare str roll + craft str roll, but this won't be dramatically better, thats why this build is so easy to progress to certain point, as gear upgrades really don't change that much, unless you get those several significant pieces from endgame section.
3. Max damage roll for 2h, according to affixes in docs seems to be +(291 to 310) to Maximum Damage, which is pretty close already.