Here's my update with Athelston, the neutral caster paladin.
I'm giving up on it.
I've made it (somehow) all the way to act 4 terror. It seems like the majority of monsters are fire immune, so apocalypse is even more useless than it already is. I'm using all the gear from the early game guide, and my vessel infinitely outshines apocalypse in every way. It does more damage, hits several times per second, and is much more reliable without draining all my mana within 5 seconds.
The jump to midgame gear (all fasc crafts) will require me to respec and invest all my points into strength (around 470) and dex (around 440). That's essentially hitting 120 and eating a fair amount of sigs JUST to be able to use MIDGAME gear. In a nutshell you have to progress to 120 and eat sigs using early game gear the entire time. Even more frustration comes from the gear selection of not only having no FRW, but getting a negative amount of FRW from one of the gear pieces. So you essentially move even slower than necessary. If you get hit by a frog, miasma'd from a gloober, and then hit with cold damage from the on-death Frog Prince skill from the monsters in kurast in A3, you're dead no matter what due to not being able to move.
It took me about an hour and a half of TP'ing, summoning 15 scourges, going full kamikaze and rushing/dying just to kill terror Mephisto. Strolling through act 4 afterwards, my vessel does so much damage that I can kite most mobs except for heretics. One single heretic will instantly melt my summons, and then instantly kill me. And this is only in terror. Even the mere thought of trying to hang with terror Diablo has taken what's left of my motivation to "tough it out" with this build. I already know right off the bat that I will get shrekt trying to vessel him to death. I'm not even sure I can kill Ghom in less than 30 minutes with this build.
I know I have more to add to my critique of this build, but I know I'm forgetting a lot too. I made it to lvl 81 terror act 4 before simply not being able to progress any further. I won't be able to hit 120 or kill Diablo/Baal from terror or further.