1 They most likely mean TORX screws, think they will be T30 or T35.
2 Best place to ground amp is at battery, less chance of picking up unwanted noise through wire this way, but any secure point will do, so long as good connection, ie under seat base to rear belt anchors ?
3 Yes
4 Best run is jusyt remove screws holding the side trim on above the sills, then tuck behind the carpet etc, replace trim.
tabetha