They're not silly questions. All of us were noobs at one point, and don't let anyone tell you differently. We all had to learn the same things, and I did it the same way you're doing, by asking someone with more experience.
Breakaways won't snap under just a little pressure, so her rubbing on the fence won't do it.
I was hand grazing my TB out in my trainer's field, when he stepped into the chain on the lead line. He jerked his head up, and the breakaway piece on the halter snapped. If it hadn't, he'd have broken his leg.
I don't normally use lead lines with chains, but I'd JUST brought him home and that was the one his former owner had given me. I hadn't taken a regular lead line with me, so I had to use that one. Had my boy not been wearing a breakaway halter, I'd have had to put him down the same day I brought him home.
So you see, even us oldsters still do stupid stuff now and again.
I no longer use that lead line and he has another breakaway halter, just in case.