Joe:
I think you may be over-engineering it. I have the Williams E-Z streets turnouts and it should be possible to make a loop on each end of your layout which lines up correctly. My turnouts are permanently installed on my layout but by laying loose sections of straights and D-21 curves above my track it appears that a loop can be made using the available lengths of straight track. Each loop would be constructed as follows:
Coming out of the straight branch you will need a total of 20” of straight track. Then you will use 5 consecutive D-21 curved sections followed by a total of 17.5” of straight track to connect back to the curved branch of the turnout.
Because the last piece of the 20” straight track length on the straight branch will adjoin a curved track, as previously posted you will need a 2.5” C-T-S connector track section at that location so that leaves 17.5” of regular straight track. Using the fewest sections possible would require one 10” section, one 5” section and one 2.5” section of straight (all narrow grooves) in addition to the 2.5” C-T-S.
But the 17.5” of straight track length on the curved branch requires a 2.5” C-T-S connector track at each end. So that leaves 12.5” of regular straight track needed which would be one 10” and one 2.5” straight section in addition to the two 2.5” C-T-S sections.
And as previously posted the outside diameter of the loops would be 16.5”.
HTH,
Bill
