If the hobgoblins can't develop an arcane caster internally, maybe they can outsource; Contract to a human or drow or other species caster, using the mercenary paradigm I described before. Someone very like Z'zdtri, ready to pitch in for a lump sum payment or a share in the magic items or revenge or just for the fun of it. It's just a matter of finding a caster who is 1) Available for hire 2) powerful enough to contribute to the effort 3) Not so powerful as to try to take over the whole show for their own evil overlord fantasies. Maybe a few mid-to-high level casters with hobgoblin 'bodyguards' who can both protect them from the Azurites while also serving as insurance should the mercs get ideas. This is essentially how Redcloak recruited Xykon in the first place.
Respectfully,
Brian P.