What are the criteria for when a (formerly) sapient creature's remains are eligible for burial?
The wiki page on Coffin (as redirected to from Death), seems to indicate citizens, caravan guards (and presumably merchants?) should be available for burial once the body has been discovered "officially". It also seems to indicate dwarven visitors from other civs would be eligible, although that may just be wordings remaining from before visitors were introduced. Also, diplomats are not mentioned on the wiki page (and I don't know if they can be buried or not).
Basically, I look for rules that can be applied to determine if someone would be eligible for burial or not.
The second question is about cause of death. As far as I can determine, there are two ways for a unit to die:
1. Being a domesticated unit that's slaughtered. This is indicated by the "killed" flag being set as well as the "slaughter" flag. No incident report is generated, so the time of death cannot be determined.
2. Anything else. This is indicated by the "killed" flag being set and the death cause field referencing an incident report containing the details.
3?. Is there any case where no incident report is generated for a killed unit that wasn't slaughtered? Given that units away on raids are removed from the fortress set of units, ones killed on raids might not get any incident reports either, but they don't produce any bodies (at least not at the time this is written), and so cannot be buried.