Re: broken world causing client-side crashes
So, we tried that. pruning everything from the map except our base didn't help, pruning the outer walls of the base did.
While rebuilding the wall we noticed an issue - we built it out of stone walls with a stone slab on top to keep spiders out. When there were fences (stone walls) in the water, one of the mods renders water in the fence block. A bunch of these together causes enormous client-side lag whenever a block is built near them (not necessarily the same chunk). We removed the fences in water, restored what we'd lost and continued on. The pruned world outside no longer had dead trees or slate in it, which was a bit problematic, but further exploration into new chunks was fine, so we could keep playing.
The next day I log into the server and insta-crash
When I get access to the save I'll try removing all the stone walls in mcedit, see if whatever mod is changing those can be avoided. As it stands, this modpack is unplayable