Pretty much. I have seen a few services pop up recently that will do this for you, but that costs money, and is of course including a third party. This is why prevention is largely better than cure.
Depending on where you live (and therefore what legal protections you have) you can demand your details. In the UK this is the right of access and is exerted by making a subject access request (SAR).
Discord was (is?) rather aggressive about setting up new accounts. You could have clicked an invite link two years ago and it associated with your email and generated an account.
Pretty much. I have seen a few services pop up recently that will do this for you, but that costs money, and is of course including a third party. This is why prevention is largely better than cure.