Try a replacement cable on the Seagate external. If that doesn't work, have a nerd with desktop computer crack the case open. Pull the real drive inside, plug it into a real SATA bay, and test it.
The external may have simply died. It happens.
error: directory valence check: directory (oid 0x3): nchildren (237) does not match drec count (192)
warning: snapshot fsroot tree corruptions are not repaired; they'll go away once the snapshot is deleted
error: btn: invalid btn_btree.bt_key_count (expected 4816048, actual 4816097)
Not sure about the first error, sounds like the directory was expected to have more entries. Probably not serious. Most likely some temp files didn't get fully written when the machine locked up.
The 2nd error can be ignored. The TM snapshot in question will be discarded at some point.
The 3rd error is probably another view of the first error. Some scratch files lost during the crash.