This
Damage Simulator should answer your questions
lvl 3 armour will not give you any extra survivial against a skulk with or without focus (4 w/out, 2 with). Lerk gives you an extra bite with or without focus. W/out focus, lvl 3 gives you one more attack against onos, but with focus, they both take 2 attacks
work the other 2 out yourself

All forms of grenade damage are increased with damage levels