How can I recover professionally from being a crank?
A few suggestions:
- If it were me, I would be very forthcoming about the illness. Of course, no one is entitled to your private medical information, so you will have to decide what you're comfortable with. But "an undiagnosed brain tumor caused me to act erratically" is a very convincing explanation, and does not reflect poorly on you.
- Consider reaching out to your previous advisors / colleagues. Even those who were furious at your previous behavior would probably accept that an undiagnosed brain tumor caused you to exhibit poor judgment. It's a "Dr. Jekyll and Mr. Hyde" situation.
- Along similar lines, consider contacting the university, and the specific school within the university, to discuss whether the expulsion can be expunged or converted to some other classification. Most universities are really understanding if you speak to the right people.
- When you apply, I would absolutely explain what happened. I would give a shorter version of what you wrote above -- even if they don't recognize your name, they will see that you were expelled, so it's better to tell your story. And you did a good job telling the story above, so I trust that you can write an explanation that is similarly clear and compelling.
- There will likely be no need to provide medical proof. You could (perhaps even should) offer to provide proof, but I suspect most admissions committees would not request it. Among other reasons, it will likely be clear from your tone that you have recovered.
- I don't think it's necessary to send an apology to your entire mailing list. But I don't think doing so would be inappropriate either. If you do so, be concise: no need for more than a sentence or two.
Change your name
I don't know about other countries but in the UK one can change their name by deed poll. https://www.gov.uk/change-name-deed-poll
It is also acceptable to write academic papers under a pseudonym. If I publish under a pseudonym, can I still take credit for my work?
Also in the UK, universities are required to take issues of mental health very seriously. https://www.universitiesuk.ac.uk/policy-and-analysis/stepchange
If you did very well in your undergraduate studies, I suggest you contact lecturers who were kindly disposed towards you then and ask if they will help by providing references. Remind them of your record (get a transcript) because, if at that time you were an 'ordinary' student they may not remember you. If they got word of your "craziness" they will only have heard it second hand. Tell them about your brain tumour and its removal - no need to mention follow-up drugs IMO. Remind them of how you were when they knew you and ask if they will support you on that basis.
Once you have been formally accepted at a new university, now is the time to notify them of your legal name change. The paperwork will go through the office.
How do I know this? I actually did the name-change thing. I didn't do what you did in terms of mental health, but for personal reasons I wanted to leave my old life behind (see note). No-one but my family and a few old friends know what my name used to be.
If you are not in the UK then of course you will need to research the situation in your own country. (or apply to a UK university)
Note: Don't worry, nothing criminal!
Congrats on identifying and moving past your illness! You'll find academics a comparatively accepting crowd as you move on.
I would suggest contacting those you've "cranked" to, telling them exactly what you've told us. You had a brain tumor, and it's been removed. You're embarrassed by your behavior during your illness, though not apologetic, as people don't need to apologize for being ill. Tell them future contacts from you will be in only professionally appropriate situations, and request that earlier contacts be evaluated in appropriate context.