PENNER, STANLEY W, is listed as a USAAF 1944 casualty here
http://www.accident-report.com/crews/1944/NAME44P.html
BCL 1944 volume gives the information that 1st Lt Penner came from California.
There are numerous reports that Americans wanting to fight before 7 December 1941 simply crossed into Canada and enlisted in the RCAF. Once the 8th AF was operational, many chose to rejoin as an American airman, with European combat experience.
Whether that was the same for Penner, I cannot answer, but a probable route.
A little more searching and PENNER Stanley Warren is recorded here
http://immigrantsofwar.blogspot.com/...interview.html
(Ctrl + F gives you a dropdown box and insert "Penner" to find the name) but nothing else seems recorded there about him.
It's likely that his body was returned to the US (California?) so it may be possible to trace him through American records.
Presumably you already found this older thread?
http://forum.12oclockhigh.net/archiv...hp?t-3401.html
The rear gunner was RAAF and sometimes the Australian records contain additional details not available in RAF or RCAF records, so may be worth seeing if his file has been digitised for review.
Doesn't answer your questions, but may provide ways to get them.