@GNRi1959, my baseboard is made of 18mm MDF and I have never had a problem with it moving/ shrinking/ swelling or warping. It is based in a room in my home so not exposed to conditions that would come with it being in a loft or shed. The frame under is 70 x 30 red deal (probably overkill). But to answer your question , no I have never had any problems with MDF. I wouldn't use WBP (water bonded plywood). It warps like crazy and actually it is never flat from day one in my experience. In my opinion the use of 70x12 battens on edge sounds okay if you use enough cross members. Hope this helps (I'm in the same game by the way)
Derek