I've heard that holding block is often used to indicate one isn't there to PVP.

But, as you note, entrance into Cyro is consent to PVP.

If you see another player you think is just there to PvE, and you don't want to engage them, don't make any aggressive movements. If they are far away, don't move closer, stop and then move in a direction away from them. If they see you, you can show your non hostile intent by doing the block animation a few times and/or putting away you weapon. You can cap that off with a /wave to show you are friendly.

That said, you will eventually get betrayed, like a lot. The other player may change their mind and get tired of you, and now you have your guard down. Before long, you will find the more prudent course of action is to either kill that player or sneak away and go elsewhere. (Invisible+speed potions are good for the latter.) If you still feel like being nice, wait for the player to grab that skyshard, kill that boss, or turn in the quest before you kill them.

Yes, I think there is. Pacifists are usually identified in Cyrodiil with a little PLUS SIGN above them. Others share this symbol, so it is not exclusive to pacifists.

usually you just tea bag

If anyone's interested I've so far found that the most effective way is simply to do nothing. Just stand still, weapons stowed.

Some people still killed me, although some seemed a bit unsure about it when I didn't react, even stopping periodically as if waiting for me to do something. (Although once I was actually afk, watching from the kitchen so I couldn't do anything even if I wanted to.) But others have hit me once or twice, or just drawn their weapons and then left me alone.

Whereas sitting, waving or bowing, the things which have worked for me in other games, all seem to be taken as a sign to attack, as does blocking.

Incidentally trying to do a Dolmen with friendly enemy players is an interesting challenge. I was in Bruma doing the PvE quests at the same time as a few other people, at least 1 of us from each Alliance, when the Dolmen started up. At first we were all fighting as normal but then I realised we were hurting each other as well as the daedra so I switched to single-target attacks only (luckily I was on a character where I could do that) and I noticed everyone else doing the same. It's an interesting change having to actually aim and time your attacks to make sure you hit the enemy and not the 'ally' next to them, but we managed to finish the Dolmen with no player deaths. Then we all stood around awkwardly for a bit like everyone was thinking 'so do we fight each other now'? Before heading off in case anyone actually did.

I'd love to see a non PvP version of Cyrodiil just to do the quests, delves and get the skyshards also to explore the region without getting ganked. Yes, I'm a weakling scrub, I fully admit it

Jumping sometimes works but never assume, particularly if you did something that looked shady.

Sheath your weapon

When I was doing my explorer title I saw 3 people in delves from other factions, all waiting at boss spawn points. I used to /dance and it always worked, we ended up spamming emotes till the bosses spawned and then I bowed and we went opposite ways. Lucky? Not sure, I was a high cp so maybe people didn't wanna try or just maybe they were like me only there for the pve. Weird that you can dungeon, hang around with and befriend any faction yet when you step in cyrodiil everyone has to hate each other.

If only imperial city had those sorta civilised types...