Chuck:
The answer to most of your questions seem to be "whatever works".
I recently installed a macerator into the sewerage system of the 345 -- Link Here:
http://www.airforums.com/forum...ight=macerator
There was one (factory) rubber double clamp splice, and the thetford valves are connected with rubber compression fittings, and adapted to "plastic" from there.
My friendly Orange Homer Despot had various types of "Plastic Pipe Glue", I purchased a type that seemed to be universal, the one white PVC (T) fitting was installed into the (Airstream Black) by utilizing the glue, and the joints do not leak.
The thetford valves are indeed held together by screws, and you can totally disassemble them for installation or maintenance.
I don't think you'd want anything smaller than 3" on the black tank, but I really don't know of anyreason you couldn't go with any smaller size of your choosing for the grey tank discharge.