I hate to give bad news, but there are lots of threads on Pond Boss with people asking how to stop the GSF from mucking up their pond management plans.

Your investment in mosquito fish and fatheads is pretty small right now. You might be better off with draining the pond and performing a fish kill? (Definitely non-expert advice from me.)

1.) Can you excavate the grade sufficiently at the end of your spillway to create a splash pool with an air gap to stop fish migration? I think that will save you lots of headaches later fighting against the d*mn GSF and other unwanted species.

2.) If the answer to 1. is yes, then how quickly would your pond refill under your normal weather conditions? If it is relatively quickly, then you might be able to purge the pond and then only be a few months behind in your plans.

Sorry to hear about your Green Sunfish invasion. Good luck winning that battle and moving your new pond into the "fun for the whole family" phase!