some questions on Go-Back-N (GBN) protocol

I was reading a textbook which describe Go-Back-N (GBN) protocol with Finate State Machines as pictures below show: enter image description here

enter image description here

where ^ mean initial state. So my questions are:

Q1-THis is the interactive animations link to GBN https://media.pearsoncmg.com/aw/ecs_kurose_compnetwork_7/cw/content/interactiveanimations/go-back-n-protocol/index.html

And let’s say we are sending packet 0, packet 1 and packet 2, and packet 1 is lost beforen it arrive receiver. So when it is timeout, GBN will make sender re-send packet 1 and packet 2, And I found an interesting thing which is,if you try the link with above scenario and you send packet 3 just before timeout occurs,this action reset the timer, because you can see the timeout is postponed(otherwise you will see packet 1,2,3 flying in screen).But according to the sender’s FSM, after you send packet 3, timeout occurs immediately, then the sender will re-send packet 1 and packet 2, but it didn’t happen as timeout is postponed, which contradicts the fact that time is only get reseted when base = nextseqnum?

Q2-On the receiver’s FSM, why it needs to set expectedseqnum = 1 and make a packet based on it? why we cannot set expectedseqnum = 0 as the only initialized action?

Go-Back-N Protocol not efficient?

Let’s say we have five packets

p1 p2 p3 p4 p5

to be sent sequentially:

and for some reasons, p3 got delayed so it the the last packet to arrive recevier.

so below is the receiving order on the receiver’s end

p1 p2 p4 p5 p3

and according to the Go-Back-N Protocol, the receiver will only send acknowledge of p2 when it receive p5.

then the receiver receives p3 right after p5, and then it sends acknowledge of p3 to the sender.

But there will still be a timeout and the sender still has to re-send p4 and p5,even though the receiver did receive all packets, isn’t this Go-Back-N Protocol really inefficient?