I’m about to start the civil war for the Stormcloaks, however I’ve read that if you go too far into the Civil War quest line there is a possible bug that prevents the treaty negotiation in Season Unending from even happening. What is the farthest I can go in the civil war and STILL have Season Unending happen?

Also, are there any benefits from stopping sooner or later in the civil war questline?

And finally, does the civil war start back up again once the Main Quest is finished, or does the treaty stop it all together?

You dony need to do the treaty.
I think either side needs to control atleast 3 holds. If they have less then the treaty cant be done.

You cant give the message from tullius / ulfric to the jarl of whiterun, once that is done, the treaty is done.

You have to stop at Morthal.