Well, Complete Mage has Fey Presence:
Fey Presence
[Heritage]
You share your ancestors' knack for playing tricks on the minds of others.
Prerequisite
Fey Heritage, character level 6th, Nonlawful alignment
Benefit
You gain the following spell-like abilities, each usable once per day: charm monster, deep slumber, and disguise self. Your caster level equals your character level.
Obvious problems include weird fey influence on your devil, and alignment issues. Both things I’m inclined to hand-wave (for players as well as NPCs), but they are there. Also requires Fey Heritage, which is basically garbage (+3 Will vs. Enchantments). I’ll keep looking.
Aberrant Dragonmark (Player’s Guide to Eberron) works for certain (Humanoid) races but not for an Osyluth (or much of anything else that qualifies for Fiend of Corruption, though I suppose a Half-fiend Human/Dwarf/Elf/Halfling/Orc could do it).
I’m pretty sure that’s it for feats.
For templates, Half-fey (Fiend Folio, same as Fiend of Corruption in the first place) is CR +1, gives charm person at will, and a bunch of other stuff that’s pretty useful (like flight). Of course, if you’re going for something that’ll increase CR, Beguiler (Player’s Handbook II), Dragonfire Adept (Dragon Magic), Sorcerer, Warlock (Complete Arcane), and Wizard all work, and I think Binder (Tome of Magic) does too.
The first edition Monster Manuals don't have a "table with monster level" - you are probably thinking of the random encounter tables from the 1e DMG Appendix C. Similarly, the random encounter tables are not found in the monster books themselves in 2e. In 2e, the DMG (Chapter 11) teaches you how to create random encounter tables rather than, frankly bizarrely, having one that applies anywhere, anytime - this replaces the stock tables in the 1e DMG.
Now. in the 2e Monstrous Manual they do have leveled Monster Summoning tables, which are similar. But none of those tables, 1e or 2e, included every monster - if you are looking for general power you have HD and eyeballing special abilities, that's it. Strict balance of encounters is not part of the 1e/2e playstyle. And you create your own random tables given what you want to populate that region/dungeon with.
Best Answer
The concept of "monster Level" is introduced in the Dungeon Master's Guide, Appendix C, page 174. The levels for the creatures in the Monster Manual are in the tables on the following pages of that appendix. The listings in the Monster Manual II include it right in the monster listing for convenience, because the book wasn't published when the Appendix C tables were compiled. As the Monster Manual II says in How To Use This Book (p. 6; emphasis mine):
Monster Level is a rough indicator of relative challenge, but is far from an exact science. Monster level is a direct function of the XP value of the creature, as calculated from the table on page 174 of the DMG. Monster level is roughly equivalent to the level of the dungeon on which they will be commonly found, though they can be found lower or higher, in greater or lesser numbers encountered.
Created as a rough guide for random encounters, it should be taken only as a suggestion, and as a means of sorting creatures into any random encounter tables you might create yourself. It was never intended to be used as a fixed measure of difficulty, and if you try to use it that way (like CR in WotC editions of D&D) you'll find your games become more unpredictable instead of more predictable.