Timing question on new Neheb's mana: Is that mana still available to cast a Madness spell I discard to hand size during cleanup?
Yes, the mana will still be available for use.
When you go to your cleanup step, you first discard to hand size and then end any “this turn” and “until end of turn” effects, including Neheb’s which stops his mana from emptying, but the mana has already floated into this step and so it’s safe until we pass into the next cleanup step.
Yes, there will be a next cleanup step since something is triggering in this one and giving players priority. Once everything is cleaned up, the madness trigger goes on the stack and then resolves as normal, allowing you to cast the Fiery Temper or whatever with Neheb’s mana. Just don’t try to repeat this trick in the next cleanup step since Neheb’s mana will be gone.