Not necessarily, but you’d need to have both cast’s and call’s return immediately with a 0-sleep, so that all messages will be received and buffered and then you can handle a given message in the 0-sleep timeout function.
In general absolutely this, but there are definite use-cases where a priority queue is very important.
You don’t have to return a message to a call immediately, you are just supposed to do it ‘sometime within the timeout’. 
Alternately promote the priority of messages-in-waiting in order to return most, if not all responses “sometime within the timeout”- which means “managing priority” becomes much more complicated and is tightly coupled to what the GenServer is actually doing.
For anyone that comes here from search like I have, Erlang/OTP 28 now supports this: Erlang/OTP 28 Highlights - Erlang/OTP
Priority Messages
Sometimes, it is important for urgent messages to skip the queue and be read by the receiving process as soon as possible. Erlang/OTP 28 introduces priority messages, an opt-in mechanism that allows the receiving process to let certain messages get priority status.
I haven’t tried to use this from Elixir yet, but it ought to work.