Only have to guess at your problem but if you read the answer above, there's a few reasons why a car overheats.
I've never heard of 100% antifreeze, it's almost always a 50/50 mixture. ON your no-heat problem, it cold have been that your cooling system was low and when it's low there's air goes to the highest point which is the heater core in the dash. The water won't circulate properly threw the core and you have no heat. When the mechanic changed the thermostat he might have bled the system properly leaving more air in the system and the cause for overheating.
Tell him to use a 50/50 mixture and also that Chrysler recommends HOAT coolant only. Also to make sure all the air is bled from the system.
http://www.search-autoparts.com/sear...27/article.pdf