[obloquy]

If balrogs were truly Maiar, they could change their form to suite their whims.

No, Balrogs (post Battle of the Powers) were incarnate Maiar, and were thus unable to change their shape. Evidence of this is that their deaths were final, and they were not re-embodied after being slain. For more discussion of this particular point, see Dragons vs. Balrogs.

Of course I just quoted that the balrogs were scourges of fire, which would give them a uniformity. Considering that Melkor's "specialization" was in fire and ice (don't ask me where this is stated for I cannot remember), it would seem only natural that this be one of their "treacherous gifts."

I doubt this. I think they were 'spirits of fire' by nature, like Arien the Maia:

Arien the maiden was mightier than [Tilion of the Moon], and she was chosen because she had not feared the heats of Laurelin, and was unhurt by them, being from the beginning a spirit of fire, whom nonetheless Melkor had not deceived nor drawn to his service.

Though perhaps it was Melkor's specialty in ice and fire that attracted mostly fire spirits.

Anyway, I have theorized in the past that the 'treacherous gifts' spoken of were probably incarnates given to these Maiar for sexual purposes. This theory ties in to their incarnation (which, if we assume was finalized sometime after the Battle of the Powers, handily explains our AAm '3 or 7' note), qualifies as 'treacherous gifts', and parallels the Biblical account of the origin of demons.

For of the Maiar many were drawn to his splendour in the days of his greatness, and remained in that allegiance down into his darkness; and others he corrupted afterwards to his service with lies and treacherous gifts. Dreadful among these spirits were the Valaraukar, the scourges of fire that in Middle-earth were called the Balrogs, demons of terror.

So many were drawn to him in the days of his greatness (Sauron, probably, but who else?), but these were not the Balrogs. The Balrogs he corrupted later with lies and 'treacherous gifts.' This theory does have a problem in the timeline. It appears that, according to the Annals of Aman, the spirits who became Balrogs were already serving Melkor at least 1,050 years before the Elves awoke, and thus before he had any such gifts to give. Still, I don't think the theory can be entirely ruled out. Perhaps they were merely promised such gifts to begin with, but didn't receive them until later.

More likely, I think, is the possibility that these 'gifts' were not necessarily 'humanoid' incarnates (i.e. the Children of Iluvatar), and maybe this is tied in with the devising of Dragons, which Melkor is said to have bred. Consider this excerpt from an Annals of Aman revision:

And in Utumno he multiplied the race of the evil spirits that followed him, the Úmaiar, of whom the chief were those demons whom the Elves afterwards named the Balrogath.

He was apparently breeding these (embodied, but not yet incarnate) spirits with something, unless he could somehow replicate discarnate spirit creatures.

In any case, I believe "treacherous gifts" must be a reference to their incarnation, whatever the means. I can't think of any other gift Melkor could give that would be considered "treacherous."