1) All it takes to start a TBS battle is simply teleporting the player to the constructed map. Nothing more. If you make those maps, you can make an event to give you a random number and teleport the player to that map based on that random number. This is best setup in a common event so that you can keep track of it.

2 A) I am not entirely sure what you mean here. The player controls what units they want placed but if you want to force actors to appear on the map, you can use the place actor plugin parameter in the map configuration to force place them. You can even make them auto battlers by giving the placed actor an auto battle state in the placement settings. You can force add non-party members but they won't appear if it would exceed the map actor limit. You can also add/remove player actors by event commands.

2 B) Yes.

2 C) If you want to, you can, instead of using numbers, use script calls to use game variables instead and have parallel events update those game variables to determine what enemies spawn.

2 D) I do not know how this would work in an SRPG?

3) This is the case for conical facing attacks when I tested them but with no move region tile flagged. Players can cast spells on enemies not in line of sight.