You have covered a lot of the things that have concerned me about the design. I think I am going to make some new tip floats and mount them further outboard. I have seen many full size sea planes and they all have tip floats in which only one touches the water at one time. I don't kow how you plan on stretching the fuselage
I set the stab incidence as per the moulded saddle. It seems to look about right and the polyurethane glue seems to be doing it's job. If I move the motors as far forward and as high as is practical I may not have a severe water spray problem. If I do I'll add spray rails.
AS you stated this is a pretty rare design and I'm not aware of any others. The plans for mine seem to be for a smaller version which was 40 powered. The instructions call for a 2-1/4" spinner but I'm sure I could easily use a 2-3/4" one. Also the wing shown is tapered where mine is constant chord.
I'll post a picture when I get a bit further along.
So many planes, so little time.
Peter