Seconds before you commented I finally realized about the armor thing lol; I didn't realize the glittering indicated armor at all on my first play. I had hesitated to use my metal units because of their low health, so despite me paying attention to stat changes, I never gave myself the opportunity to understand it absorbs a hit (to make that understanding leap I would have ALSO needed to have actively noticed both the particle effect, and its absence afterwards, which I clearly didn't in the case of Dwarves ). I like hidden mechanics conceptually so I get the intent, I just totally missed it here.
The space management stuff I understand would be pretty complex to "fix" here in a way that prevents it from ever happening in the first place, but I think there might be some ways you could handle the "error case" more cleanly. Referencing other merge games I've played, for cases like this I think they usually have a "bubbling" objects that are being forcibly given to the player when they don't fit on the field (they go into bubbles which float above the grid, allowing you to "pop" them when you do have space). Dunno how well that'd fit here, but something to consider.
Glad I could help, and your inexperience doesn't show at all, you clearly know what you're doing in terms of presentation and a lot of the little UX things developers frequently overlook.