Not being an aerodynamicist, I can imagine, that the Payen layout would give similar characteristics to a
tandem wing design, so getting a bigger marging for the CG. And for a medevac aircraft, where two additional
passengers should have been transported horizontally, this could be an important feature, I think.
So I would regard the patents for the medevac designs rather as recommendation for a plausible use of his design,
than to be taken literally as a construction plan. If they could have achieved had STOL performance, I'm really not
sure, but the mentioned types with that feature were designed later, I think. Other contemporary designs probably
lack STOL performance, too:
https://www.secretprojects.co.uk/threads/latecoere-late-20-ambulance-airplane.28097/
https://www.secretprojects.co.uk/threads/delcourt-flying-ambulance.15318/#post-96932
"Medevac" probably wasn't meant back then to carry the wounded away directly from behind the front line !
About computer modelling, yes, it could be a way to determine, if a design is plausible at all. I just use it from
time to time to the proportions right of a design, I only know from a perspective view, like those drawn by
Roland Payen himself (attached example should represent the Pa.57). And yes, I sometimes had the feeling, that
his sketches somehow whitewashed his designs, often making them look more slender (e.g. the Pa.61). If another
view is available (in my example even the original flying hardware), I'm keeping to it, of course. If no other, more
plausible or realistic sources are available, I would keep to the available source, even if it is clear, that this design
would be less, than ideal, to say the least. Our definition for a "project" is, that it was drawn with the intention
to be built, or at least drawn for "serious" reasons. If it would have been workable, is another matter. I'm pretty sure,
if all projects, that would in reality never have been able to leave the ground in that guise, would be marked red here,
you could print out many sections and used them as a red flag ...