Is it illegal to subsitute Torx (or square-drive) screws in
certificated aircraft? If so, why?

Nope.

Okay, that said, why is it that everyone (that I've seen, anyway)
continues to use the (too easily strippable) Phillips screws?

Inertia? Cost? No one has thought of it? No one else cares, cuz
their A&Ps do all the work?

It just seems so bloody obvious, yet I've never seen a certificated
plane with anything but Phillips screws.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"