4) Any optical connection will have a connection loss.
A loss of 0.5dB per connector, or 1dB for 1 connection (two connectors) is a
worst case spec for most ST connectors.
From my experience typical is 0.5dB per connection.
The fiber-optic feedthrough is just a glorified mating sleeve, and other
than it gets dirty because Transformer OEM forget to put the caps on, I
don't see them having more than ~0.2dB extra loss.
In short, 10~20% loss is not surprising for a ST-ST connection via mating
sleeve.