I just received a 2.5" doing 6 clicks on power on and then failing most ATA commands. Am I to assume this means it hangs in bootstrap firmware because it can't read any on-disk firmware or SA? Clicks obviously mean something bad about the heads not being able to get anything from the platters for whatever reason (i.e. head damage), but the fact that it doesn't hang on ABRT seems to be somewhat strange?

Since this is a 6 year old drive, the common defects are probably known, is it known to have failing heads?

disk returns status information. (Special error code) 
it can be obtained from nvram

Does it do standard TTL UART to get that info?

no. uart not used for this purpose. ata-ordinary style of work