Am I allowed to double lug (a circuit breaker) for a hardwired secondary surge arrestor?

Considering the fact that the main disconnect is a suitable overcurrent protection device, I could connect the device to the bus bar lugs. The wire size is too small, so how could I fit a #12 wire into a lug designed for something like #1 wire?

Are wire connectors allowed in a panel? I'm replacing a panel and would like to know how to lengthen existing wires if the connectors are not allowed.

Panel is mounted against the wall and the ceiling is low (6.5 or 7ft). Can I bundle the cables (with wire wraps) coming in overhead through one large 2in. knockout and through one cable clamp, or must I bring them in seperately, in conduit? The distance to the ceiling will only be a few inches.

If I mount a 3/4" piece of plywood directly on the masonary block wall with powder actuated fasteners, will 3/4" long fasteners in the plywood alone be adequete to carry the weight of the panel?