Disconnectiong you wallet is like logging out of your account. You can do that by clicking on your address on the top right corner of the page and clicking on the little "Disconnect" icon.

Leaving the guild means that you will lose all your roles and rewards that you collected in the guild. You can only get back to the guild if you can fulfill the requirements for at least one of the roles and you will only get back your previous roles if you can still fulfill the requirements for those.
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For example, if the requirement is to "Have xy role for 1 month", the counting for that one month will start all over again if you leave the guild and try to join again.