You should not need any seal or putty behind the light fixture. Certainly need a new gasket for the fixture though.
Ground is so soaked here, I'm not sure where my back wash water is going to go (other than the neighbors yard... eventually).
Where do your downspouts drain to? I plumbed a 4"x4"x1.5" wye into the 4" underground line from the downspouts and ran a 1.5" line to my filter. Be aware I'm not sure all municipalities allow using their storm drain system for this purpose.