As note spell-alike abilities CL is based on your HD. Anyways your CR 19 with pleasure devil which is Demilichfiend. You have 8 class levels then to choose (but your 12 HD). But if you take +2 more CR for +4 more HD (and so become 16 HD creature), you can then get 6 levels as fiend of possession and so be 22 HD creature (yay for one epic feat).
Actually, CL is only based on HD if it doesn't have a listed CL in the monster's entry. In the Pleasure Devil's entry, it's listed as CL 17, so that's what it stays at, regardless of whether or not the Pleasure Devil's HD increase.
For the Lichfiend template, I'm allowed to qualify without spellcasting if I have at least five at-will Spell-Like Abilities and a CL of at least 11. Meanwhile, becoming a Demilich requires Level 21 spellcasting. The Pleasure Devil has nine at-will Spell-Like Abilities, so following along with the Lichfiend's prerequisites, I'd be able to get the Demilich template if I raised the CL of my Spell-Like Abilities to 21, if you decide to allow it to work that way.
After that, I just need a way to increase the Caster Level for my spell-like abilities by 4.
I was either going fiend of possession or using a cohort with it, brachina was one of the creatures i was looking at too.
Ah. Well, if you don't want to double up on a prestige class like that, I could certainly fill in that slot, if you'd like. Or one of us could use the Fiend of Corruption prestige class instead...