Not usually. Everything piles up underneath and gets slowly composted into the soil. Those sort of trees don't have very big branches anyway, so there's not much big litter. Just a constant falling of the needles.

It's more than likely moss, not grass.

It's commercial woodland. They're planted in rows as it's easiest for the machinery to deal with. The trees grow up as they have to compete for light as they're so close together, there's no point growing branches low down, as that wastes energy.

It's not particularly biodiverse, but it's often better than most of the other profitable uses for the land in an environmental sense.

About 70% of the stopping power comes from the front brake. It's why higher end bikes generally use bigger front rotors compared with the back. Same thing with motorbikes, they usually use a pair of large front disks.

The rear brake really only slows you down, it's the front brake that stops you. You just need to shift your weight back as you stop, otherwise you will fly off the front, it's a simple fact of physics.