Can you confirm that Adapt content will also send n=0 if it is the only element?  E.g. cmi.objectives.0.id  as opposed to cmi.objectives.id

That's correct. Specifically you can see that happening here.

it was trying to set cmi.core.total_time

Adapt (out of the box) definitely does not try to set this - as you say, it's read-only. Adapt will only try to set session_time. Perhaps this was added as a customisation to the course you are testing?