The gas cap turning the light off for a time indicates you may still have a small air leak in the emission system. Could be a line on the charcoal canister. Have you been given a fault code? P0447 is a general evap code, that in most cases won't effect drivability.
Any loss of power? Plugged Cat converter on older units is a concern.
Broken vacuum lines on the heater control box may be causing both the no heat & SES light concerns.
If this doesn't help I can only repeat what I told Customers when they called the GM store for help...hold the phone a little closer to the problem.
Good Luck
Bob
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