So, I'm playing a do-gooder in anticipation of Dawngard and get the quest to find our what happened to Ranmir's old flame Isabelle. Like just about everyone else you go looking for she's dead. Fine. I'm getting used it. I get the letter, go back and give it to Ranmir and crush his drunk heart.

I go off on more adventures and eventually return to Winterhold (I'm working on becoming Thane everywhere - because I'm clearly bored). Ranmir is nowhere to be found. So... Is this a glitch or did my bad news push him over the edge? He's not at Birna's and he's not in the Inn and he's not in the Jarl's Hall. Frankly, there aren't that many other places to look.

He could be dead. He often dies during the containment quest for the mage's college and his body can be near the inn or the store. If it has been a while then hte body disappears. If you haven't done that quest yet then maybe a dragon killed him.

I like keeping my npcs alive as well but some are just not worth it or you won't gain anything from them being alive - like Ranmir.

Yep. That happened in one of my games. His body stayed for quite a while, but eventually despawned.

No dragon attacks around Winterhold yet and I'm not doing the College quest line... I just can't find him. Weird.

I wonder if some NPCs just despawn. In my 2nd game in Riverwood the blacksmith's wife was killed during a dragon attack. The blacksmith 'disappeared'. I assume he was killed in the same attack, but I never found his body and Riverwood ain't that big.