why is בך not accepted, only בכם?
It is correct you can report it
hem tzofim bakhem (m.pl) / bakhen (f.pl) / bekha (m.s) / bakh (f.s)
What is the difference between:
הם צופים בכם?
הם צופים אתכם?
The difference is that הם צופים אתכם doesn't work in Hebrew. After צופים only ב is used - הם צופים בכם is the only correct solution.