1) Keep branches a few inches long, perhaps they'll develop into fruiting spurs. Trim anything that gets more than 6 inches long. I prune non-scaffold branches to 5 buds in the summer, then 3 buds during the dormant season.

2) Your staking plan sounds just fine.

3) Rabbits will eat the bark off the lowest branches of your espalier during the winter. The rabbits in my yard aren't desperate enough during spring, summer, or fall to touch the fruit trees. I wouldn't use the white plastic thing during the growing season, just during winter. I need to cover the whole tree in hardware cloth during the winter, because the snow lets the rabbits access most of the tree otherwise. They can eat anything from zero to 2 ft high.

4) Your soil pH shouldn't change much, and it's a bit silly to worry about it after planting the tree. But you should invest in a pair of nice hand pruners. Don't prune with kitchen scissors.