Mulching should not have caused the demise of the Kirengeshoma and the aucuba. The Selaginnelas being low-growing would not appreciate a deep mulch. I would mulch the Kirengeshoma and the aucuba.

Kirengeshoma is the plant that could appreciate extra water in a summer and wouldn't mind seasonaly wet feet in a spring/winter, so I'd not attribute its demise to the leaf mulch.
I have it in 3 locations in my garden, all three are mulched by cedar mulch and I see no ill effect whatsoever, even in location where water could stand for 7-10 days in a spring.

Someone I know swears she heard that oak leaves are poison to plants, and she picks each one laboriously out of her flower bed underneath it. Not sure of the truth of that.

I can't imagine anything killing Kirengeshoma palmata; koreana maybe but even it is pretty vigorous. Are they fully hardy for you? Maybe they died for some other reason.

Oaks have a lot of tannins, but generally should not be a problem for most plants. The K.
palmata is hardy for me in my Zone 4 garden- have had it for 8 years.

The leaves that you put down might have attracted voles that feed upon your plants. Unless you
put the mulch done over 1 or 2 inches deep your plants should have been alright.