You're probably right about double sided tape on those frames. The frames are cheaply made with glued mitered corners with staples in the corners - tacky....
Could be they might have used something like shoji-nori glue, but who knows, maybe even Elmers, for all we know.
PVA might be a better choice than RSG if not in a strictly enforced climate controlled environment. If the environment is controlled RSG is best as it will pull up tightest. Prints can be sprayed on back with distilled water, if the paper has been sized with RSG prior to printing. PVA is non- hygroscopic so would work well in all situations, so you're right that would be a good choice.
There are several archival DST makers, namely Scotch, 3M, and Universal.
All have archival iterations of their DS Tapes.
Sometimes Hinoki (Japanese Cypress) is used for frames because of the tight growth rings and excellent grain.
Normally it has few knots and is light weight with straight grain. Additionally, mostly withstands water well.