But, unfortunately, there's still one problem: you can make the gray ball also generate the sub-tile elements (ya know, the DF and MM walls), which does not only create them in their unnaturally big size, but also turns them into a deco (pretty much like char elements, which just have a center as an obstacle). I think the best solution for that is rather just make gray ball create the 16-th state of each mini wall (the full set of four walls, much like when you place them in R'n'D engine, it just fills the entire tile with all four of them at once) or just prevent it from creating them completely.
Fixed -- will now be created as a four-wall block.
(BTW: Did you really manage to use them in the R'n'D engine? I only get question marks then, as expected.

)
Also, nitpicking, sorry, but... could there be an option of changing gray ball's melting behavior to just custom shrinking/exploding animation in the future?
I added a checkbox "[ ] explode instead of melting", which does just that.
What might also be useful is a transparent ".changing" animation (or ".shrinking" or ".opening" or whatever it might best be called), that would be drawn on top of the new element during the animation (similar to the current hard-coded "melting" animation), as the ".exploding" animation could be customized, but is drawn instead of the new element, changing to the new element graphic after the last explosion frame (just like in the R'n'D engine when blowing up a wall with emerald, for example).
Speaking of mini tiles, I once tried applying a custom sprite for the empty space (via custom graphics, ofc), which does work in MM engine, but only for the full empty space tiles. I know it's a minor thing, but, making it just use the empty space texture probably won't work well because it will shrink it for the x16 tile (or, hopefully, draw behind the wall sub-tiles), so what if instead we could do custom textures for each MM/DF wall tile state? I mean so we could apply different full-tile textures for two neighbor tiles, any set of three of four blocks or full 4-block set.
These are in fact two ideas.
But I do like both of them. Not sure how easy or hard it would be to implement this, thought.