Hello, and welcome to the forums!
Enemy Lure is usually good for a few extra levels if taken early and you fight and defeat literally every encounter possible. However, it is worth noting Enemy Lure also helps with finding more player ghosts (a good source of powerful modified demons, if you can manage to recruit them after dealing with the ghost) and 'cryptid' demons which are otherwise extremely rare to encounter. So while XP is definitely a major benefit, there are other perks too.
Summoned enemies never give XP: not my favorite design choice ever, but otherwise you'd be able to easily farm XP from "protect" style links and Demon is designed to have a finite amount of XP available per game, rather than offering grinding/farming as options, so it was a necessary semi-evil. As you noted, you do get some XP for successfully completing the link (this is true of all links, incidentally, not just 'protect' style links.) The XP award for recruitment is the same as for defeating: this was actually done for the same reason as the "no XP from summoned enemies" bit: Demon offers a finite amount of XP per game and I didn't want to penalize players for one of the key actions of the game, namely, recruiting demons.
Each level requires a set amount of XP that escalates as you go. When you get a kill or successfully recruit, you and every demon in your party (active or not) get the exact same amount of XP, which is based on the defeated/recruited target's Level and Rank (rank covering business like Heroic, Minion, Normal, etc.) There is no penalty to the amount of XP earned for fighting lower level demons, or any bonus for fighting higher level demons, but since each level requires more and more XP, you will naturally see less progress towards a level from defeating/recruiting lower level targets, and the opposite for higher level targets.
The game is balanced such that the player character will roughly gain 1 level for each dungeon floor that is fully cleared. I specify player character there because your demon allies have one additional rule: their levels not only take more and more XP as levels increase, but they also incur an additional XP requirement per level away from their "starting" level: this being the level they are *normally* found at, which may not necessarily be the level you recruited them at if they came from a corpse or a player ghost.
Generally speaking, this extra "distance from starting level" penalty begins to become difficult to ignore at +10 levels from where the demon started. This exists to strike a balance between getting to enjoy a well-crafted (or RNG blessed) demon ally for a fair amount of time while also making sure recruitment continues to be an important consideration throughout the game.
Lemme know if you have any other questions.
I admit it is pretty quiet around here, but I still check regularly in case anyone stops by.
Cheers!