"...Maybe the design decision was not about structural strength of the screw..."
I actually made some calls today about this. Had a nice discussion but to summarize:
If the access panel has a proper doubler, the screws do not need to be structural as the doubler is carrying the loads.
If the skin has a pre formed joggle that the access panel sits in, structural screws are technically recommended as there is no doubler to carry the loads.
Structural screws have a minimum strength of 125,000 psi
Stainless screws have a minimum strength of around 75,000 to 85,000 psi
All that said, this is experimental aviation and everyone is free to choose...
This post is ONLY about passing on information...