Hi johnep,
You are very welcome, and congratulations on a good quality/value latex mattress from Arizona Premium. 
It is really quite clear that you have done your research and testing, really well done.
I understand what you mean about testing for alignment when you are by yourself…not that easy to do, but given you are fairly light and haven’t had many issues, and with your extensive local testing it looks like the three-layer design you chose has very good chances to meet your spinal alignment and support needs.
Regarding the foundation question … it would be important to check with Arizona Premium to make sure that the support system you plan to use for the interim period wouldn’t invalidate the product warranty. From a general point of view, you can place a latex mattress on very different surfaces …on the floor, plywood, slats, traditional box spring, old box spring… but each time you do this, it will feel different as the foundation itself will add another variable. With this in mind and if Arizona Premium vets your support system to use for a month or so, there should have no problem using your old box spring with this mattress until you are ready to get the adjustable foundation. Many consumers have done this with no issues at all but I’d first make sure to check that the support system is still in good condition and does not sag under the weight of the mattress and people sleeping on it. When you change to a new foundation there will be some “feel difference” which may be very noticeable, may only be slightly noticeable, or it may even feel much better. There’s much more information about the functions of foundations, active slat systems and actual box springs in the foundation thread here.
There is more information about choosing an adjustable bed in post #3 here and the main adjustable bed topic that it links to that can help you choose an adjustable bed based on price vs features comparisons and also includes some retailers that you can use as good sources of information about the features of the adjustable beds they carry and as pricing references as well (in(in post #6 in the main adjustable bed topic).
Good luck when you start down the adjustable purchase cycle.
Thank you for the great update.
Phoenix