Not losing Celestial spirit - I suggested a tainted scholar route. I agree that 18+ levels are necessary if you are going the ES route. Paladin already has medium armor proficiency, so proficiency from ES is irrelevant.
You might want to work out the AP before commenting, because it's not as clear cut as you mgiht think. Compare the suggested AP from slarden (41 ES, 7 TS, 32 SE). If you just give up trying to fit in nerfed ES, you have a lot more freedom to develop your build how you like. There are a lot of strong enhancements at T5 souleater that you want: supreme hunger, feed on magic (more ruin spam), spell tearing.Gain:..
nerfed Paladin saves.. Nerfed or not, it's +11-14 to all saves
not enough prr/mrr to cover what you lost in ES Debatable....
ASF spell failure. Not sure why you think there is ASF
I realize 20 warlock ES build is likely to be more damage, but it's not at all obvious that it is significantly more damage (misses out on TS crit damage and some T5 SE enhancements) AND you lose out on A LOT of defensive power. A lot of quests are essentially unsoloable without no-fail paladin saves (or are at least much more difficult).