A:AnswerThat is not one question :-[
You should be able to add a 2nd receiver of the same brand (and sometimes other brands).
You should be able to use the security feature across multiple devices (sender - receiver 1) then (sender to receiver 2).
You should not need the security feature at home.
You never get more speed out of the system than it is capable of at the sending point.
If you have 300 Mbps and 5 devices are actively using it ... they will be sharing that speed ... like cars on a road.