Many Evoker skills can serve as tools in one’s toolbox; grabbing a few as 1-point wonders may be useful. The fire tree alone may not be ideal though, so if you want to use 1 tree and curses/utility skills you may want to get such trees as Lightning.
Yes, those that already deal splash damage do.
+5% radius, +8% power, a few others may be suitable.
Yes. +1 to all skills can be augmented into any helmet that’s above ~lv20, and there’s also Schaefer’s Foul Recoil (an early-game pistol) and Hocus Pokus Locus (a mid-game focus item) that have this property.
Since it doesn’t synergize well with crit builds and the majority of endgame builds are crit ones, probably not.
Crit chance is very readily available in the endgame, so nothing prevents a combination. However, elemental strength is only really useful against Dessy, while crit is globally useful.
All skills that deal damage should be able to crit, yes.
Indeed. Night Striders (the endgame set boots) provide more than one so, coupled with +1 to all skills, you could end up with more for a reduced CD.
Tempest is not similar to the other two, so I assume you meant Lightning Field. Hellfire may be inconvenient due to its fuse timer, so between Field and Swarm I’d suggest Field due to its CD/power cost; it can be stacked, unlike Swarm, which can make it deal more overall DPS.
Nothing preventing one from using more than one though, especially since Galactic Conquest (the endgame Glyph) provides all 3.
Ember is a 1-point wonder meatshield. It can be useful early on, and I personally use it for a while. It can very safely be skipped though, as it’s not really endgame viable.
Probably not, unless one specifically intends to do a “phaser” build; dedicated Spectral tree and Elemental Drain, with matching gear and mods. Not sure how viable it might be though.
Spectral Serpents are still one of the best Evoker skills however.
For more info on the class, feel free to go through these