I can't see him doing it without Mariah, if its struggling to sell WITH her it'll be worse without.
Lionel normally does well as a live act, struggling sales never seem to have be an issue for him in the past. If anything, Mariah is the weaker commodity when it comes to tours. Her stepping out and him reducing the prices to compensate might do a world of good and bring his usual crowd of pure Lionel fans.
Maybe for both fanbases the tickets really are just a little too high for only an hour of your fave, no matter how much you love them.