I used Permatex High Temp Form-A-Gasket Sealant at the threaded joints. It is resistant chemical deterioration, will withstand temperatures up to about 600 degrees and is designed for gaskets, flanges and connections. So far, not a single drip.
The swing check valve simply prevents back flow of exhaust fumes and sea water to any portion of the exhaust system in front of the valve. I suspect most plumbing supply houses sell these. It is basically a cylinder with a hinged flap inside that swings in only one direction. Slight water or exhaust pressure applied against the flap opens it mechanically. Water and exhaust flow out the exhaust pipe with no risk of back flow when the engine is not running.
As for galvanic concerns, I have a number of sacrificial anodes installed on the boat...on the propeller shaft, water pump, heat exchanger, etc. So far, I experienced minimal deterioration. The preexisting swing check valve was made of brass and showed virtually no prior signs of corrosion. Brass has a significantly lower anodic index value than does aluminum or zinc, meaning the risk still exists for my brass exhaust system but it won't go before the anodes. If corrosion ever becomes an issue, I will install a galvanic isolator to save my alumimum anodes and other metal parts that come in contact with water.