Clash-of-clans – Help the friend leave his invisible clan

Since you have linked both villages with gmail accounts it shouldn't be a problem, and you cannot actually replace a village with another using the same gmail. So it should be safe for you to switch between the two.

I am saying this because I linked a new village (by mistake) to my email account and was trying to replace it with my real village but it won't let me (even COC support couldn't help me and they suggested I create another gmail account to link it)

The main use for clan tags is to use it when searching.

When searching for a clan with a common name, it can be hard to find the exact one you are looking for. So Supercell added the 'clan tag', for making identifying clans easier.

You can search for clans by their clan tag in the search tab.