Okay, now I have a real plan (though obviously it's not finalized - that's the purpose of this thread). Let me know if this sounds reasonable. Remember that I have problems in my shoulders, neck and arms. I think the lower back problems I have had are clearly from the mattress, not the accident. So I want to concentrate on making the mattress soft for my shoulders and arms, and just "normal firm" for my back.

Here's the plan:
1" layer of VERY FIRM HR foam on the bottom right on top of the springs of the Fenway Sealy. (My question here, is: Is 1" best or would 1/2" or 3/4" be enough? Or two x 1/2" layers just in case I want to experiment with just one layer, or two?)

Then I'll buy enough HR foam in various densities to experiment with 2 x 1" layers of HR medium and firm foam over the bottom layer. That's 3" of foam total. Then, over that, IF needed, maybe the 1 and 1/2" memory foam layer OR just one 3/4" or 2 x 3/4" medium soft latex as a comfort layer.

Then, I plan to divide the bed into 1/3's and buy enough pre-cut pieces of 1" HR foam to be able to play with 2 x 1" of HR foam layers for a total of 2" over the bottom firm layer.

Or should I go with 3/4" for these 2nd two layers for a total of 1 and 1/2" instead of 2"?
I'll buy these layers in pieces cut for zoning in Firm, Medium Firm AND Medium, so that we can play with the placement of all those: firm over medium, medium over firm, medium over medium, etc. depending on the zone. For my shoulders I'll probably actually use some of the 2" soft latex and/or memory foam that we already have.

If this works at all (i.e.; if the mattress feels anywhere near "good" over those springs, with these various layers of HR foam) then after awhile we'll experiment with some Dunlop latex. Oh, and also I'm buying one of those latex "toppers" from Brylane in a Queen size so we can cut it up for zoning for the twins.

Comments? DIfferent ideas?
Thanks. Had it not been for you guys I'd have never thought of taking apart the Sealy! And hopefully it will turn out to feel good once we get the right combination of foam over those springs!

By the way, the Sealy sewn-on topper fits right back over the foam on its "hinge" (where we didn't cut that side) and it wouldn't take much to create a velcro seam to fasten it down with, or something like that...