Invader/invadee etiquette

There isn't any.

If you are invading expect the host to flee and heal if he is low on health. You are forcing your way into his game. To get upset because the host used estus would be like getting upset because you broke into someones home, waited for them to call the cops, then get pissed off and butthurt because you got arrested.

If you are the one being invaded, expect the invader to use everything they have to kill you including the enemies and other invaders, if the fight turns against you, run to the nearest bonfire so you can easily pick up your souls. if you cant make it to the bonfire, run and find a good spot to hide, heal then pray the invader doesnt find you. If he does, hope you get lucky and can beat him. Don't be afraid to summon help and dodge and run until they summon in.