Hi Barney,
I don't think that Jamestown is that well known nationally but locally they are "hopefully" better known. They are owned by a father and son team .... both named Jim (confusing when you don't know which you talked to

) and in my conversations with them I was impressed with what they were doing. When they do use polyfoam ... it is higher density and quality than the "stuff" used by the national brands. You probably saw this but they have an outlet right in Warren.
I would tend to pass on the Restonic ... partly because it is unlikely to have the same value as a local manufacturer and partly because they are made differently (and named differently) by each licensee in different areas of the country and often use a couple of inches of poly over the latex (as in the the Interlude you mentioned). Polyfoam in the support layers under the latex is not nearly as big an issue as it would be higher density and firmer and not in the "weak link" area of a mattress. Without a spec sheet that shows every layer such as one of my favorite Restonic licensees who shows all the layering on their
website here ... I would not likely consider them since I have personally experienced too many sales people telling me and others that you are "sleeping on latex" when you're not. Of course if you get all the specs and the mattress is perfect and less than anything else available (very doubtful) then of course I would consider it.
OMF has only 2 latex mattresses ... one is a 6" latex core with an inch of poly on each side and the other has 1.5" of convoluted latex and an inch of poly on each side of the same core. Given the wider latex options and value that is right in your own city ... I doubt I would drive that far for only 2 possibilities.
Unglued layers in the Savvy Rest (which makes a great quality layered mattress but is overpriced IMO compared to other available options) and others don't present any problems that I know of. They have been used for many years and the cover is tight fitting and the latex doesn't tend to shift easily. If the mattress is moved and it does shift, it's easy to unzip and shift it back. Durability is not an issue as long as the cover is well made and tight (latex breaks down with ozone and ultraviolet light and a good ticking is important). I also like the idea of exchanging layers and side by side constructions where 1/2 layers can be exchanged ... particularly for an online purchase ... however its not as important for a local purchase as actual "lay on mattress" testing is usually more accurate than an online purchase.
Most local manufacturers will change out a layer if your needs change years down the road and sew the cover back up. Various more specialized construction techniques such as tufting can also be done with "finished" mattresses that are not available with zip covers. The worst case is that a sewed cover can be removed and old layers can be put inside a new zip cover purchased separately along with any new latex layers you wish to add or replace although glued layers can be difficult to take apart in this case.
Phoenix