Assuming I am understanding this right, and you want a separate button for Mitigation and another for HP restoration, you have doubled WAR's button presses to mitigate something that other jobs only need 1 button for. This might or might not be something that appeals to people, but to also note, it restricts the available buttons for rotational needs.
However, WAR would sill be in a weird spot. You have more HP, but you still need to survive the initial hit, so you need to have that minimum mitigation, of which you can then heal after. The question is, how much do you heal? Too little and you make healers work harder, too much and you negate healers. We also have to consider that increasing max HP means it takes more to heal, so, unless you give them a healing increase, you also have to take that into account, which makes the calculations even harder and much more volatile. Now, HOW that HP is healed is then another factor, do you tie it to damage, or, do you make it all like equilibrium, where it is a fixed potency. Even more variance.
We have to remember that original WAR from 2.0 was a high HP tank with very few mitigations, intended to be a take a hit and then heal it after. It didn't work.
Yes, still have the same considerations that blocking has, which I also neglected to mention won't reduce damage from DoTs, just the initial hit (another reason why Holy Sheltron was changed). Just to point out, block went through the exact same thing, never used to be able to block magic, now it does (as of SB). But it is these changes that initially had all these flaws, which, over time, was adjusted, which is how we ended up where we are now and the 'homogenisation' of tanks. If we were to start changing things too drastically, you will find flaws, scenarios where something doesn't work, the community will ask for it to be changed, and we will end up back here. It is an inevitability.
I'm not going to go through everything, but expending HP to do your job won't work, evasion is broken and parry isn't reliable (and making it 100% just makes it a pseudo % mitigation).
As for balance, it is clear you haven't put any thought into how it would affect the jobs and the balance to go along with it. Which is an issue a a lot of people have. Sure, you could throw HP expending things on DRK and hope it works, wait for the playerbase to complain about it and then do a soft rework in the next major patch, but you can avoid all of that by just thinking, how would this actually affect gameplay and balance. Unfortunately, most people would rather skip the critical thinking and would rather throw things at a wall and see what sticks.
Again, healing balance, and again, you do still need to survive that initial hit.