I'm a few quests away from Season Unending, but I'm curious:

If I choose to go through with the peace treaty instead of picking a side, will the war start up again afterwards (leaving it open for me to pick a side then) or will that be an end to it (leaving no more civil war stuff available for this playthough)?

Cheers.

Bump - really would like to know so I don't miss out on stuff.

Yes, the war rages on, so you can do the MQ then the war.

Thank you. I appreciate the help =)

at what point is it to late to get peace?

If you have chosen to complete all the Civil War quests first to their conclusion (and hence wiped out one or other of the two factions) then it's impossible to make peace (because one faction is dead).

Prior to this, if you even start the CW quests for either side, then the Season Unending quest becomes bugged (or so sayeth the UESP wiki).

This is only partly true... The bug (caused by a dialogue loop) will most likely happen if you started the Civil war line at the end of the 2nd stage of the MQ. or the other way around. To avoid.. best finish either Season unending before entering the civil war Quests... or make sure you finished the first phase of the Civil War Questline. (ie up untill the battle for Whiterun)
Be aware that the Civil War Questline will be locked (because of the peace treaty) until you actually finish the main quest. If you finished the Civil War Questline.. Season Unending will not even be triggered.