A little more update.
I did an experiment by using only the PS/2 keyboard and mouse (without connecting the USB ones), as I realized that USB stuffs don't work on floppies that could boot and with their own USB stacks.
1. coreinfo now outputs one of these two errors instead of the "Not enough memory creating EHCI periodic frame list." I used to get, although the first error message only appeared once. Not enough DMA memory for OHCI HCCA. drivers/usb/usb.c:42 new_controller(): Failed to malloc 588 bytes.
2. At one time I managed to make memtest enter SMP mode. However, I only did the first test, as during the procedure (address test, walking ones, no cache) I got errors every 2GB past 32GB, with the good and bad ones showing inverted contents (bad is ffffffff when good is 00000000, and vice versa).
That was the only time I managed to get memtest to enter the test phase. With other attempts (and without entering SMP mode), I got the same glitched screen.
So it seems there are issues with both the memory configuration as well as the USB-related stuffs. Unfortunately, it seems large RAM configurations (like 8-16 sticks of 8-16GB) were only tested to a limited extent so there might still be some bugs that don't surface with small RAM configurations (like 1-4 sticks of 4-8GB) which were tested more often.