Funnily enough, I was looking at this the other day, and it reminded me of a recent - quite lengthy - debate about N18:
What is the probability of rolling a "natural 1" on a D3? Examples: Stimmer's Combat Chems Stash & Wrecker's Jump Booster. Bonus question: Can you roll a "natural 6" on a D3?
yaktribe.games
Fluff-wise, it would make sense that a Yoof doesn't dig as much as a Boy, so is less likely to cause a cave-in. However, I think the RAI is that a 6 is a 6, even if it's halved.
I don't have a specific source in relation to the mine, but look at Digganob p. 17. Digga-boys and -yoofs only generate d3 income, yet there's a special rule (Nightmares in the Dark) that triggers on three or more 6s. So, there they must be counting what's rolled on the physical d6 rather than the simulated d3.
I don't think they make any difference.
Orks don't actually get pinned anyway. So, I believe the falling when pinned rule was only introduced in Digganob (p. 12). There it simply says if you're about ground level - and gives the wall of a fort as an example.
There are various other (non-pinning) cases where someone is knocked off a vehicle, e.g. because they lose hand to hand combat (Roolz, p. 43) but those conditions don't say anything about edges or safety railed - you're simply thrown off.