This site is a wealth of information. Thanks to all.

Based on the explanation, I must have an EZ Sport, as I have 26/20 inch wheels. I contacted the bike shop that has the eBay listing and they tell me the seat they're selling is steel. I think I'm going that rout, rather than trying to find someone who can fabricate a seat for me.

I really don't know about exact measurements, but I think the seat is the right distance from the pedals, as determined by the bike shop guys (Touring, Tandems, & Recumbents of Greenville SC). I don't put a lot of stress on the handlebars, because they will move some if I do (my main complaint about the bike is that you just can't tighten them enough to be fully secure in place). As to the amount of stress I put on the seat as I pedal, I can't imagine any way of reducing that pressure and still being comfortable riding the bike. In fact, when I had to ride it the rest of the way home on the day it snapped, I noticed just how much I "slide" back as I pedal without that support to keep that from happening. I just can't see how I'd get any force on the pedals if I didn't have some pushback on the seatback.

Anyway, I only ride about 5.5 miles each day on the commute, all on city streets, so I'm hoping the all-steel replacement will work out for me. The bike shop guy in Florida (eBay sale) tells me it lists for $95, so I figure $45 is a reasonable deal (plus shipping) and then I'll have a lot of other extra parts around should some of the other pieces fail.