You're clearly getting the hang of this thing!

It's pretty clear evidence that the 12V supply suffered a nasty energy spike when the BST opened up - I guess they don't particularly care what happens to the car electronics after that point.
That part is also a bidirectional TVS as you have worked out. This one is a Littlefuse
P4SMA27CA P6SMBJ27CA, again a 600W 25.7V (27V typ.) part. It's probably a case of different sub-assembly suppliers having their own favourite parts / supplier pricing arrangements. Yes the SM6T30CAY will be fine.
Oh BTW, this is my favourite site for smd part identification...
http://www.smdmark.com/en-US/search/code?id=27ca I usually look up the full part numbers on the web rather than relying on the embedded PDF datasheet links. Maybe just me being cautious.
P.S. Hopefully the ECUs used bigger TVSs, they are usually much larger package parts.