I believe "lineup subject to change" is meant for people who are purchasing tickets, not the talent who should already have these things in their contracts.
Either the promoter promised both they'd be headliner or one of them is full of shit (and my money is on Monique being full of)
No way in hell she headlines over D.L. which is evident by that poster which clearly features him more prominently than anyone else.
I would say that if this was in any other venue, D.L. would be over Monique. However, Detroit women rock with Monique. She has a dedicated fan base that stood by her hard. It's hard for me to think that she wouldn't be a ticket seller there.