You mentioned finding 15 diamonds in chests, but I've found a total of zero in both game modes after digging hundreds of thousands of tiles and opening hundreds (if not thousands) of chests. I've collected nearly a hundred diamonds total (and used them up as well) but they were all from digging or jackpots. Is it even possible to get them from chests?
Yes, it's possible, but very luck-based. Diamond chests start appearing from depth 250 onward and can spawn all the way to the end. However, their spawn rate is about 1/5 that of titanium chests. It might make sense to slightly increase the spawn rate while lowering the number of diamonds per chest, just to make the experience feel a bit more consistent.