Warlock/mage is the best warlock combination for endgame because it is easily the best "support" class in the game. You have surge of awareness (20% attack power increase), Warp Charge (party wide attack speed/cast speed increase), soul brand for fire and dark damage dealers (mostly warrior/mages), a 10% patk boost through Otherworldly Whipser, can double the damage dealt by 1 attack by potentially all raid members with Silent Seal, a crit resistance debuff, AND you can res players in battle.
As for skills to max, well just read above. Save some points for your damage spells for questing and whatever else, but maxing at least surge of awareness and warp charge is mandatory (they're too strong!).
