Sorry for a long reply too. About mrc.bin: no, it's actually possible to use mrc blob on Sandy/Ivy, but as I see it's not supported across all boards. X220 has support, other boards needs patching (or maybe patches are already on gerrit, I'm not sure). It shouldn't be hard to get it working, though.
Can you elaborate on this one? Why does the X220 has support and other Sandy/IvyBridge based laptops are not supported? Wasn't one of the ideas for coreboot to have a more common code base or am I missing something obvious?
Basically, it's just not enabled in other boards' configs. I don't know why exactly it is enabled only in X220, but I can guess it's because the native raminit is preferable, works quite well and nobody needed mrc raminit on other models. You can try this patch https://review.coreboot.org/c/coreboot/+/37153 on your T430.