The arm could have a hidden crack around the area that holds onto the square spindle. I would inspect it carefully. As you already know, it can cause a crash if it fails at the wrong time during a ride.
Given it's the left crank arm, it makes it easier for a quick swap if you have a spare from another crankset laying around. It would be curious to see if it still becomes loose, even with a new crank.
If the arm is not cracked, then maybe thread locker on the bolt should help a little bit. I've seen that new cranksets come with the threadlocker pre-applied on the bolts when bought new. Even if the arm becomes loose anyway, the threadlocker should at least prevent (or delay) the bolt from undoing completely and the arm falling off.
The spindle could also have a crack. Might be worth inspecting it too.
Being left outside, the elements might have accelerated corrosion of the interface between arm and spindle. I would inspect that as well.
Personally I would just replace the BB and at least the left crank arm, just to be safe from crashes.
Could the stiff pedal have anything to do with the crank arm becoming loose? I doubt. Let's imagine the pedal is completely locked, bearings as stiff as if they were being welded in place. Then you push down on it. What torque is produced, and where? Firstly, at the pedal thread, in the direction that undoes the pedal, so the pedal would become loose. And secondly, at the square taper interface, in the normal direction the interface was designed to withstand torque, right? My intuition might be wrong, but I don't see how the stiff pedal could cause the arm to loosen.
Another posibility could be that the bolt can no longer push the arm on the spindle far enough. Why? Because the surface on the arm that contacts the bolt becomes flush with the side of the axle (where the bolt screws into) too soon. Why? The bolt could have chewed away too much material from the surface of the arm. But this is very unlikely with a steel crank. Even more unlikely if there's a washer under the bolt.