Can a Terrapin pick up the trail of an approaching Eclipse (both in their default configuration) in time to warn a nearby fleet?

It's possible, but it won't be easy for either ship. Terrapins are extremely strong at detecting signatures and whilst the Eclipse is very good at having low signatures, it still has one. With a Terrapin in a good location, with an operator on their toes they should be able to detect it but whether they have time to get the message across is a different problem entirely.