Personally I didn't have good luck with the Blauparts kit, but it seems to be only me that had the issue with the driver's side valve cover that I got in the kit. (They're interchangable) I eventually got an OEM gasket and that did the trick for the passenger side. Luck of the draw, I guess. Everyone on the forums here seems to swear by their valve cover kits. I would give it another shot for the price.

As for smoke coming out of the tailpipes, is it white or darker black smoke? Leaking valve cover gaskets usually don't result in smoke coming out of the exhaust like that. They simply allow oil to leak down onto your exhaust components and burn up within the engine bay.

I don't believe a heater core issue would cause you to actually hear hissing when you go to fill the system again. If you're thinking it maybe a cracked heater core, you would also notice your coolant level going down and your car beginning to overheat gradually as it got lower and lower.

Have you had the car in service position recently? If not done carefully, you could break some of the hoses that are used within the AC system.