that we never had seen Demandred under an alias on-screen.

Darlin has been around for a long, long time.

And again, why would Demandred care about an army of non-channellers anyway?

His top priority is Rand, and matching Rand in battle.

Darlin's army wouldn't stand five minutes against Rand's channellers. Same for Roedran, the Bordermen, the WC.

Demandred is behind Taim, and was the only Forsaken clever enough to build a real power base that would come into play only in Tarmon Gai'don, meanwhile putting an expandable pawn between Rand and him. If things went sour, Rand would strike at Taim, not at Demandred, and there was always an army of BA he could fall back to then.

Demandred's main goal is to defeat Rand. He wants to kill him himself or finish him off on the battlefield, and nothing else would do to satisfy him because it won't prove that Demanded is the better man of the two. To achieve that, Demandred needed Taim's faction, and after the Cleansing he's realised he needed a mix gender army to face him, thus the BA proposal of allying with the BT.