I am not a hardware expert either. Wren was the name for the disk
device in fs(8), and I think kfs inherited the name. It refers to a rw
disk or partition. The messages can be understood AFAIK as I/O errors,
which usually correspond to broken hardware.

Regarding 2, I don't know how that may be.
Regarding 3, I know disks that do automatically what time ago you did
by hand (declaring some blocks as defects and instructing the disk to use
other spare ones instead). The only reason I may find for this is that your disk
detected an error and was able to correct it; but I'm not a hardware expert
either.

A power failure may cause all this depending on the disk you use, because it
may lead to broken disks (although I admit I've not seen this since long ago).

We use Plan 9 here for daily work: It runs a lab for students, our accounts,
we write programs and documents on it, read mail, etc. I indeed can say
that it's more reliable than Linux, according to my experience (that can be
different for others, of course). So I'd not be scared to use plan 9 for daily
work; I'd be to switch back to what I used before.

If I may help somehow, let me know.