The other option - which worked better in Dark Heresy - was the Daemonvessel. The subtle difference to a 'host is that a 'vessel is in control of things (ish).

Essentially, it's more akin to Malus Darkblade - you have a daemon (controlled/voiced by the GM) in the back of your head, poking you with sarcasm at inopportune moments. You can, in emergencies, allow it to manifest a portion of its power through you - strength, psychic power, etc - but every time you do, the cost is another slice of your own soul (and a bundle of corruption points), and an ever-increasing chance that the daemon will refuse to go back into the bottle once he's finished...

Equally, he is visibly human - only when he lets the daemon out does he sprout claws/fire/noodly appendages

Don't deal with out-of-game problems in-game. Your player has different expectations for the game and different wants and you need to talk to them about it.

I don't, this was the resolution we came up with. I decided to throw it out there if anyone else wanted to try it. It does work very well with these sort of players.

If it works well with your group and you're willing to put the work in, more power to you. The Elite Advance and the short talent tree you have created look nice to me. 500XP for those characteristic boosts seems cheap to me, but I haven't run the numbers.

Though given the description of the problem - a player who just wants combat and not the rest, I would have taken the lazy route and given them a vicious, barely civilized Feral Worlder with low Fel and no social skills. Just as anti-social and combat focused, less work and worry about game balance. But that's just me and I'm getting lazier and lazier as a GM as the years go by. As I say - more power to your group if this fits there.

Can't really say I like the idea much from an RP perspective. It's just so one dimensional, ruins subterfuge and is a creature that's seldom used. It kind of ends up like grey knights or some of the ascension advances in my opinion, as in: these are not characters that socially interact with people around them and are usually only unleashed when it's time to rip the enemies of the emperor.

If the group is okay with it though, sure. Corruption would be thrown around like candy though, after all they are mingling with a daemon.

I'll have to Nth the badwrongfun thing. Pocket-sized warp drives made to order by friendly mechanic type Ad Mecs clashes less massively with the setting than PC daemon hosts in my opinion. But hey, you're welcome to them. Won't interfere with my games in the least. So...

If I was looking to do it, I think I'd take a very hard look at both BM & RH. RH has piles and piles of stuff on more or less heretical Acolytes and their owners (including PCs on a death countdown), and BM has a revised and expanded version of the faith "magic" system from IH, which is where I'd start to look for inspiration for daemon host powers.