It depends on what the panel manufacturer says you can do. Some can support two wires per terminal, some can't. ;

The position of the ground and neutral bars to the breakers that are protecting the circuit is immaterial. Some panels don't even have separate bars on the left and right. Some panel installations have bars for different purposes. Be very careful if this panel is not also the service disconnect, the ground bar and the neutral bar are distinct and must remain separate.

Most boxes have knockouts in various places (top, bottom, left, right, back) to make them versatile. Frequently you don't see the side ones used on a flush mounted panel because the studs in the wall (the panel is precisely 14.25" wide to fit between the studs) get in the way (and it's too much hassle to bore them out when you can use the holes in the top.

You usually can add additional ground bars to a panel.