Hmm...based on your criteria, a 'simple' solution in my opinion would be to make your warlock out of a cleric/mage build. Either dual or multi - your choice. I'd add the following amendments:
- Edit the weapon(s) of your choice wieldable by this class via near infinity
- Swap to needed weapon proficiencies via Shadowkeeper
- Edit the armour(s) of your choice usable by this class and allowing spellcasting (near infinity)
- Limit yourself to pure fire damage spells, demon summoning and 'symbols'/'powerwords', by not memorizing/scribing anything else
- Set your paperdoll to what you wish in SK - the standard might not suit you if you wish to wear armour.
Balance issues:
The unrestricted weapon selection, I don't see as unbalancing, as for instance staff of the magi is one of the most powerful weapons around - usable by cleric/mages.
The armour (plate types) are not all that much better than the best elven chainmails, and offset by all the defensive/protective spells you won't be using.
If anything, I'd say you'd be making this more difficult for yourself.
You could offset this by making a custom spell that lowers fire resistance a la Firkragg's. Possibly of type (cleric/arcane) and level which you would otherwise not have any fire spells for.
If you want a melee character a bit more tough than a standard cleric/mage with no defensive spells (suspect he'd be killed quickly otherwise), you could add some HP and an extra attack in SK.
For effect, (now I'm starting to ramble), you could edit your weapon to grant a fireshield upon equiping. Add 50% resistance to fire as innate, and your fire resistance is taken care of (alternatively, use equipment).
If you don't want that animation on constantly, de-select the weapon when not in combat. (who runs around with a sword drawn all the time anyway?)
I'd personally probably add the weapon to Firkragg's horde or the one of the demons outside the Planer Sphere. I find it more interesting to fight for your gear than buy it at a shop. Not too many Warlock outlet's around anyway in Amn
All in all, I'd say it could turn out fairly interesting...