I'll take a stab at this unless FilterGuy gets back first:I do not quite understand what you mean by 'no way to safely use a polarized breaker for the string over-protection device', sorry English is my second language.
A polarized breaker would be connected so that the normal current flows from panel to controller (or combiner). This is the easy part and works for single strings (not parallel strings).
As we all know, when you have 3 or more parallel strings you need to fuse each string for potential back feeding (to one string with all the amps of the other parallel strings combined). In such a back feed, the polarized breaker has current flowing in the wrong direction which can produce the results seen in the video in post #15.
I'm sure FilterGuy will correct me if i got this wrong. I'm also certain he'll have more to add, he's a smart guy.