T.G.B.U
I think you are right in that the only Catholic Church is in Corfu Town. I don't know if they do a Sat evening mass though.
First thing to do is buy a "Corfiot News" when you get here. This should have some info in for you when you arrive. The Corfiot is an English monthly newspaper so I'm hoping it will have the info you seek.
( And, if not , you will have some contact tel no's in there that may help )
Second thing is, maybe hire a car for three days as this is much cheaper than having a taxi each way, just for one night.
Stay Well
Negg