Background: I've been working a few paygrades above what I'm paid for about 15 months now, and made significant contributions for my team and my division (i.e. stepped in for a drunken manager and sales engineer when they nearly botched a $2,000,000 contract due to talking like used car salesmen, and saved the contract, even though I'm not a sales engineer and didn't get any commission;
I also overhauled a large piece of our main tech we sell to customers to be "Docker-ready", trained a few employees in its use to "spread the knowledge", significantly benefited the company, and automated a lot of our products and processes).
My boss decided to finally "promote" me to "senior engineer", but told me I'd basically have to work my butt off for another six months (i.e. "we like to test people in the new role"). I get all the new work and responsibility, but no new title or pay. I told my boss "the company already got three times that trial period the past 15 months: time to 'buy WinRar.exe' and the time is now to provide the remuneration adjustment). My boss declined.
After taking some time to think on it, I resigned yesterday, so my last day of work is Jan. 6th, i.e. my notice period occurs over the holiday break). I also enabled a "publish my new job information" option on LinkedIn so my new job (was able to line one up with a cloud services provider rapidly) was announced to my colleagues, and all the people I trained have jumped ship, without my deliberate influence or communications.
Problem: my employer is bad-mouthing me, and my boss keeps sending e-mails to teams outright stating that I've "deliberately sabotaged the division for personal gain". Most people I speak with messaged me on Slack and IRC, and noted that they trust my decision and believe I had good reasons for leaving (even though it means products will have to be shut down and layoffs will be a certainty by Feb. 2021).
Also, the company is making me work (oddly) during my notice period, though I generally ignore most e-mails on vacation days, rather than just paying my "garden leave to go away". They're also trying to extend my notice period another two weeks (due to the holidays) and another eight weeks due to "fiduciary duties" (trying to get me to sign paperwork I refuse to sign, since they will only provide the papers in-person, and not via e-mail, for some odd/nefarious reason).
While the company has a "final farewell/email" policy for my last day, there's nothing stopping my from e-mailing teams now (while I still have e-mail access) and informing them that my resignation was due to a dispute between me and my boss, and not a lack of consideration for my colleagues and teams I helped. Should I send a pre-emptive "farewell, and here's my reasons (because my boss is unethical)" e-mail to the teams that will be impacted in the new year? There are several people my boss is bad-mouthing me to, and I don't want to lose their respect (some of them even provided references for me in the past)?