A constitution only matters if there are measures that punish violators. In the UK the monarch doesn't want to speak up if the government violates what is commonly seen as constitutional, and parliament is partisan so the government can do what it wants.
IIRC, that's how an unwritten constitution is changed: someone does something without serious challenge, or some new thing that's demanded becomes so entrenched that there would be serious challenge to change it.
If all the opponents of "the recent proroguement of parliament" could manage is "tutting," they made the action constitutional by their inaction.
Not exactly. If there's enforcement, it will be done by other actors in the system will cooperate to enforce those "customs and principles." There's no appealing to some higher authority for enforcement (and especially not criminal law, which would be a lower authority).
You misunderstand what a constitution is. It's not a document, which is an inanimate thing that cannot actually do anything. Rather, it's a set of customs and principles that are respected and followed. The latter is what actually matters, the UK has it, and it takes precedence over any written document (e.g. the constitution of North Korea "guarantees" free speech rights, but the customs and principles of the North Korean government voids that).