Thanks for the suggestions, everyone.
I currently let SeaBIOS handle the video card's initialization, as there's currently no other options to choose from nconfig/menuconfig regarding graphics init and framebuffer mode for this board. It's just that this "Include generated option rom that implements legacy VGA BIOS compatibility" option might cause issues. Disabling it fixed most of the video issues and it's currently working fine.
I currently don't have BMC installed but I do have the module with me. Is there an up-to-date build guide for OpenBMC on this board? The build guide on Raptor Engineering's site looks outdated as some repo links were dead/moved when I tried to follow it. By the way, I'm currently building coreboot images from latest Manjaro (Arch-based) just fine (as the toolchains needed were also built), can OpenBMC (as well as necessary toolchains) be built here as well, or that I need to setup a VM using an earlier, recommended distro for the purpose?
As for serial console, I think I'll disable it once all other issues are solved. Currently I can always reach SeaBIOS boot menu with my current memory configuration, and that some of the floppy images are booting just fine. It's just that the secondary payloads (memtest, nvramcui, coreinfo, tint) are having issues.
It seems Mike Banon's patch for enabling support for multiple floppy images does work and I could now see and choose all the floppy images I put into the image, but other patches (including one which properly modified the boot selection part so I can select boot entries past number 10) could not be applied out-of-box and require some manual adjustments, even on SeaBIOS 1.12.1.
I think existing RAM HCL entries which corresponded to Libreboot versions may not be considered reliable, as some memory configurations that worked with Libreboot do not work here (which was also the case for KCMA-D8 that has been discussed before).