When can you continue the war after the truce? Will either side pick it back up only after you made Alduin "as dead as you can make him", or can it be started back up once you have Odahviing trapped?

I want to see Ulfric in Sovngarde, but I kinda also want to do the truce negotiation again. I already did the truce once in my previous playthru, so if it's either/or, I'll send Ulfric on his merry way this time.

Also, I recall seeing a screenshot somewhere of a flashback or something of Ulfric assassinating Torygg. Was I just incorrectly recalling or is there such a thing?

After you did the truce and after all the main quest bullcrap is done you can continue with the civil war questline where you left off or you can also start it. Its a "temporary" truce.