This response will not be helpful. (What an ironic start to the year!)

> i just enlarged the sda and sdb partitions underneath /boot, which broke
> their raid1 association, and then learned at grub rescue that the
> centos7/grub2 mbr indeed looks for the raid device, not just an underlying
> partition, so i wonder if sda were dead if perhaps it might still just
> work, if all i do is tell the bios to boot sdb.

Regarding "enlarging" there is no breaking of the RAID association. A given
software RAID setup done with Linux "md' (multiple devices) will read the
header block of the partition/file/drive and determine its association. If it
is inconcistent in size, it may reject it. But regardless, you can attach any
type of device to it -that is "greater than or equal to" in size-- and it will
re-build the component it needs to get to a non-degraded state. Of course you
need to be careful and know what you are doing so that you do not have
unreasonable demands of md...

GRUB is very smart. It is like a fully-funcitonal OS. Just like the kernel
can use specific RAID devices using the UUID, so can GRUB. But that does not
mean you should let it decide what you want to have happen... The two-stage
booting procedure of GRUB can be edited as you go; I am not a GRUB expert, so
I keep a lot of notes on using it that I do not remember off hand. As a helper,
I keep README files on the boot (say "/boot") partitions with descriptions,
partition tables, and lots of procedures, complete with GRUB commands, so that
I can bootstrap myself easily if I need to work with failed drives. The idea
is that you can list (ls) and read (cat) files from GRUB, so you can recover
a system without having to have another functioning system with a browser open.
(Years of experience working with failed drives speaks...)