i think the short short answer is: an example of what sender can do is here rfc6675
First, TCP receiver does not process SACK fields. SACK, as ACK, is sent by the receiver, i.e., receiver creates information in SACK, and is processed by the sender.
On the receiver:
The actions of the receiver on duplicate packets do not depend on SACK option. TCP receiver ...
You are confusing a couple of things. MTU is Maximum Transmission Unit, and it is a value of the data-link (layer-2) protocol on a specific medium.
IP, neither IPv4 nor IPv6 has an MTU. They both have maximum packet sizes, which are much larger than any data-link MTU. The total packet size (header and payload) for an IPv4 packet is 65,535 octets. For IPv6, ...
Loss of SYN/ACK (2nd step) means no side has received an ACK = there's no way to send a segment.
Loss of last ACK (3rd step) means that the listener (server) isn't fully synchronized but the initiator (client) is. If the initiator then sends some kind of L7 request and includes the ACK flag, the situation is healed.