ATM 18" f4.6 with a 2.6" diagonal

[Carl Woebcke <carlw@telis.org>]

OK.  Now with Newt30 I'm armed and dangerous.  My holy grail is to
design an f4.6 (or so) 18"er with a 2.6" diagonal (for higher contrast
and lower coma) with 100% illumination at the eyepiece.
Question 1: What's a minimum and a desirable 100% illumin. DIAMETER? (I
know, you're going to say it depends.)
Question 2: How far into the tube can I stick the base of the focuser
without appreciable (or any?) vignetting?  I'm guessing a 19.5" tube ID,
and hoping to put the base in 0.5" (-0.5" for the "Tube Thickness"
parameter in Newt25=Newt30), or 0.25" from the projected primary edge.
Question 3: Is there a focuser in the house that I can get away with a
total of 2" for height PLUS travel??  If this depends on eyepieces, then
what focuser and what eyepieces?  Mel Bartels thought I could get away
with 1.5" to 2" from the base of the focuser to the focal plane using a
Novak short height helical focuser.  Since small diagonal scopes seem to
crucially depend on a low focuser base to focal plane distance, does
anyone out there have any experience or ideas with this?
Question 4:  Am I dreaming?  Is this still Kansas, Toto?

Thanks,

Carl