Does the clarified butter "seal" atop salmon rillettes really do anything to preserve? How long can I keep it in the fridge?

Yes, it keeps the salmon fresher...really!

There's a Thomas Keller recipe for salmon rillettes reproduced on many sites.
One says they must be kept refrigerated & used within two days of breaking the butter seal.
Before that, with unbroken seal, ok for 3 days in fridge. Or ok frozen, seal unbroken, up to two months.
Have a look for yourself:
http://whatscookingamerica.net/LindaSandberg/Appetizers/RillettesAuxSalmon.htm