#cleanup step

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.

Players don't get priority in the cleanup step unless an ability triggers because of something that happens in the cleanup step, such as Archfiend of Ifnir triggering. When something triggers in the cleanup step, that trigger will go on the stack and players will get priority as normal. There will then be another cleanup step after the first cleanup step. The turn won't end until there's a cleanup step in which nothing triggers and players don't get priority.In the scenario you describe, Archfiend of Ifnir will trigger for each card you discard. Those triggers will go on the stack and resolve one by one. Generous Patron will trigger for each creature that gets a counter from Archfiend's trigger, causing you to draw that many cards. When the new cleanup step starts, you'll have to discard to hand size again, which will trigger Archfiend again, which will trigger Patron again, etc. This process will continue until either your opponent has no creatures left that can get -1/-1 counters or you lose from drawing your whole deck.

After.

The ending phase of the turn has two steps: the end step and the cleanup step.

Jin-Gitaxias triggers in the end step and you don’t discard cards until the cleanup step, so you’ll have those seven cards you drew in hand at that time and very likely need to discard a few.

Ensoul Artifact falls off.

In the cleanup step, when Inkmoth Nexus’s animation ability wears off, that effect is no longer making it an artifact creature. Ensoul Artifact continues to make it a creature, but since it’s not an artifact anymore, Ensoul Artifact isn’t enchanting a legal object and thus Ensoul Artifact is put into its owner’s graveyard.

You discard all at once.

Yep.

Because Gitrog is triggering off of the discarded lands, players will receive priority in the cleanup step which will then be followed by another cleanup step. So you’ll draw a card when the trigger resolves, then at the beginning of the new cleanup step, you’ll have to discard again. If you discard another land, you’ll just rinse and repeat until there’s a cleanup step where nothing triggers.

So… COMBO TIME: SOI EDITION #3