Hi Westcoastj.
Welcome to our Mattress Forum! 
Good to hear that you are an “on and off 3 years” old timer on TMU. Thanks for your kind words and specs and background. And now to your challenges and any “flaws” in your thinking that we may find.
First off you have a good starting point as you managed to do some in-store testing and you also are gained a great deal of clarity about your likes and dislikes in terms and material and the feel you are looking for and what you want in general.
You seem to have oriented yourself well in terms of motion transfer. While two XL Twins would work great to minimize any motion transfer you might want to take into account there will be a gap between the two units if you want to cozy up to your husband. There are some solutions to deal with any gaps between the two mattresses but you most likely will need something like this or a topper to “bridge the gap”
It looks like you are living in Canada and I don’t know how the cross border shipping is working these days but before I’d decide on a mattress I’d make sure to make a phone call to each of the US companies you narrowed down to and ask details about shipping to Canada. I see that Arizona Premium is now shipping to Canada but you may want to ask about logistics waiting times and shipping costs involved especially if things don’t go as well as you hoped for and you need to do some exchanges or returns.
Bolsa as a choice of spring unit seems to be the best for your needs as it will offer more support and less motion transfer than the standard Caliber Edge.
[QUOTE]1. Mattress Decision
I’m struggling between choosing a DIY and or one of the ready-made hybrids.[/QUOTE]
If you are looking into building your own DIY mattress out of separate components that are purchased from one or several different sources then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have realistic expectations and that you are comfortable with the learning curve, uncertainty, trial, and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
For Arizona Premium
[QUOTE]A. The Twin XL Eco option with Medium support for me and Soft for him. Seems like an unquestionable deal re: value. But it has a 3" topper. Questions on this one:
o I’m wondering if that 3" topper isn’t going to be too thick for my stomach-sleeping?
o Can someone confirm that in the Twin XL it has the Bolsa system which seems to be better for us than the QE Combi? [/QUOTE]
You did not mention what the other position of your combination sleeping is. You mentioned that you like to go to sleep on your stomach but can you determine what percentage of night you sleep prone and what percentage the other position?
Arizona Premium has a king
Echo sleep in king & queen sizes can be upgraded to a Bolsa, but neither the full or XL Twin have this choice.
[QUOTE] B. For DIY we’d go with the Twin XL Bolsa, and then two additional layers. o Question on this one:
We’re thinking the Soft for me and the Supersoft for him for the top layer, but aren’t sure about the transition layer below. How thick? How firm? [/QUOTE]
Their DIY component page has the Bolsa only for Queen and King sizes which makes me think that they may be out of the Bolsa for smaller size beds. I’d keep in mind that with the Arizona DIY option you won’t have the ability to return any DIY mattress components if your choice is not adequate as Arizona has a strict “no return policy” on those. The trial periods and 20 years warranty are for the pre-configured mattresses.
About the transition layers question it depends on your answer to my previous question.
Luma’s 8" Bolsa coil support system would be a little less firm than Arizona’s 6" Bolsa. They do ship to Canada but you’d have to ask the same questions about exchanges, returns to and from Canada, etc.
SleepEZ 360 Hybrid Pillowtop gives a choice of Talalay or Dunlop Latex with their Bolsa 8" Elite Edge Pocketed Coils. There seems to be an error on the Select Hybrid product which I’ll bring to their attention tomorrow. Also, I am not seeing options to ship to Canada at this time.
[QUOTE] My biggest question here before committing is how much lessening of motion transfer are we sacrificing by having an adjustable frame vs a solid wood platform or slatted frame? A lot, or a little?[/QUOTE] ……
Increasing distance between sleepers and moving to a king-size as you are planning will result in less motion transfer. A foundation or adjustable unit under your mattress would certainly contribute to having better motion isolation but it would depend on the type and force of the movement and on the thickness of your mattress as well (a thicker mattress would isolate you more from the springs). You may wish to review Post 18 about motion isolation
Sprit king refers to splitting the layers with the ability to fully customize each side of the mattress. The mattress encasement is surrounding and holding the layers of the mattress together. Most people don’t notice a difference in the center of the mattress unless there is a large variation between the comfort configuration on each side of the mattress.
Two XL Twins will still add up to a king mattress and can be put in the same frame but they will act as independent mattresses each within their own encasement. These are good for people who desire the least amount of motion transfer and have wide preferential and need differences. They can be used on separate box springs, foundations or adjustable power bases but can have a common frame around both units.
All in all, I think you are on the right track but would need to do a bit more leg work and decide if it is cost-effective for you to buy something from the US in case things don’t go as well as you are hoping for.
Phoenix