If I have a longer fork (with the +50 offset), how would trail increase by more than 5mm (5.3mm by your figures)?
It doesn't increase the trail, it reduces it.
Wouldn't it be on the other side of that equation and be more in the 3-4mm range? Maybe I'm thinking about it wrong, but with fork length being longer that should compensate for some of the trail differences, or so I thought.
You've sort of got it backwards.
If the fork offset were the same, this would increase trail. Increasing the fork offset REDUCES trail. The offset puts the axle CLOSER to where the steering axis intersects with the ground. The offset is what's decreasing your trail in this instance.