Whatever your theorycrafting tells you man, but I did that and boom, easiest run ever.
You can keep doing your maths, this stuff is battle tested.
And actually, early game hydra isnt about endurance adn vitality, but about attunements and spell damage. The crystal golem is a bit more requiring in terms of endurance tho, but still needs all those slots and damage.
But of course you can cheese hydra with a bow, enough arrows and lots of patience.
Dunno man, I set up to do a sorc build and Dark Souls became easymode skyrim. Just relaying my experience to other people. You found other ways to do it? Great, whatever, but every road leads to Rome.
And from my experience I found many enemies that required two soul arrows but went down with one soul spear. For a fragile caster two casts is a risk - every cast leaves you open to attack. Having to push two soul arrows means finding the window, casting, rolling around, repeat. For those times when I really needed to dispose of an enemy, soul spear was invaluable. It's not about the mag multiplier, its about removing one enemy when you need that to happen. Same for soul mass.
And you minimize it, but the window between quelaag, all of anor londo, and a good chunk of the archives is a very challenging area for fragile characters since enemies tend to start hitting really hard. Being able to use a single opportunity window to remove one enemy was invaluable.