Can you save immediately before entering Verdistis? The game will disable saving if it thinks it is in the middle of a cut scene. After taking a couple steps in Verdistis, the guard should wake up and start a conversation. Does this start and end properly for you?
As for the crashing with Kistandalius, try deleting all of the files in the '
..\Divine Divinity\Run\Dynamic' folder.
This folder is used as a cache for the game. Files are created / copied there from the save game folder when you load, and written / copied from that folder when you save a game. The files will be re-created as required the next time you start Divinity. If there was a corrupt file in that folder, deleting the files should fix the problem.
After this, try starting a new game and see if the problem persists. Use
iZakaroN's SaveEditor to bump up your constitution and run to Ars Magicana, save and try talking to him (and maybe see if you can save in Verdistis). If not, try your latest save. If the new game works, but the latest save does not, there may be a problem in the saved game folder itself. If neither work, it may be a problem with Divinity, in which case re-installing might help.
Troubleshooting and Manual Install Procedure 
