I know they have to be operational at ceiling for ETOPS but haven't heard they have to able to start at ceiling, especially considering the main engines wont (in most cases) start at or around ceiling. I know of atleast a couple airlines start the APU on ground for ETOPS due to starting issue mid air, it's cheaper to just have it running from the gate than descend for APU start then re-ascend to cruise. What I know for sure is the NGX's APU wont start too far above 25000ft

On the 737 in the US and Canada are West Coast-Hawaii routes. Like you said, it's just cheaper and easier to leave them running up until the EXP. However, not all flights enter ETOPS right away, eg. Seoul-Urumqi. In that case, the APU would be started 45 minutes prior to EEP Also, there is the APU On Demand program, which will allow airlines to use the APU on demand if they pass certification. That certification requires the cold soaked APUs to start at cruise above FL290. Did you try multiple starts?

Did you try multiple starts?

Sure did and I broke it lol