It is still unclear how horcruxes would fit into the HPMOR universe given the premise that souls do not exist. Here are some thoughts and conjectures revolving around the postulate that horcruxes are complete memory/personality backups:
To recreate one’s identity, there has to be more than just replication of memories. There also has to be a way to replicate the ‘software’ that governs our thought processes. Removing the limitations and shortcuts that our physical brain has in place would undoubtedly remove lots of mental biases (making us hyper-rational) but also raise the question of whether the resulting entity will really be ‘us’ or just well approximated copies of us (a well explored topic in the transcendent pony fic).
The very possibility of possession (as evidenced in the zombie Quirrell) gives evidence in favor of the possibility to project one’s mind outside one’s original body and still be able to retain the capacity to not only have semblance of life (a la magical portraits and ghosts), but to also be able to learn new information and retain it (i.e. grow) and interact meaningfully with the environment.
Assuming away the whole issue of whether one’s identity can be virtually encapsulated independent of our biological hardware, this would require careful copy control (lest there be multiple versions of you running around, raising the question of which one is the ‘real’ you, explored comedically in one of robst’s fanfics). Is Harry an incomplete horcrux, retaining Voldemort’s post-transcendent software (hyper-rationality), but not his memories, i.e. a failure of copy control? (He clearly didn’t inherit his rationality from biological parents?)
Magic in potterverse directly interacts with mind. So magic is capable of storing information, including memories, software, etc?
Since Voldemort has already gone through the process, he already knows what’s involved.
My thought is that wizards are not confined to projectile weapons. Armor would be next to useless if the offensive magic, for example, is fire based or involves water or gravity manipulation. Moreover, an armored helmet significantly constrains both visibility and mobility, which may make the wearer more vulnerable.