1999 Pontiac Grand Am Gt overheating

I have a 1999 Grand Am GT the problems i am having are
1.The car begins to overheat, it usually gets up to 205 but now that its been 80 degrees in MN the car has gotten up to 220, the remedy i have used that has been able to bring the temp down to just over 200 is to turn the heat on full blast.
2.The coolant/antifreeze reservoir perpetually goes dry, i can not go a week without adding another jug of coolant.
3.When i turn the car off and get out the engine smells sweet. And to go along with the smell their is a milky white substance that has condensed on the oil cap.
4. The underside of the coolant reservoir cap has a crack that looks like it is permanently pushing the spring up. would this cause the cap to release the coolant when it really shouldn’t?
We are not sure how many miles the car has on it, but the previous owner estimated it to be `Well over 200,000`. The previous owner also informed us that we shouldn’t turn the ac button off.
Does any of this go together, or is it just a lot of problems that a 200,000+ mile car has to deal with?