hello from another long term EC member – hmmm – in any complex there is typically one toxic psychopath
who enjoys making life hell for others – the question is – is it you, or someone else ? 
a lot depends on whether the malicious owner lives in your complex or elsewhere, and whether you think they are more likely a psychopath or just a meddlesome worrier with too much time on their hands.
tampering with mail – is a federal offence – I’d address that – ask them directly why they think you were, and clarify any misunderstanding – presumably as secretary you have to receive mail addressed to the Owners Corporation and may have a separate letterbox for that?
embezzling funds – again a serious offence – same question – all meeting agendas/minutes/decisions/accounts/receipts should be on record for perusal on payment of the standard strata search fee (like $20?) – do not spend time on general accusations – ask them to specify their concern – put up or shut up.
EC should present a unified front – discuss the problem at an EC meeting – put a note on the record discussing the concerns and the resulting decision – on public record for the other owners to see now and in future to dissuade this person from wanting to stir up trouble again.
tenants kicking your door – call the police over the threats – get a police report – send one warning to the agent/owner – next incident – eviction – required due to continuing breach of standard by-law 1 – ‘must not make noise at any time … likely to disturb peaceful enjoyment of another resident’ – kicking your door and calling you a c##t certainly falls into that category – see p.38 (and Notice to Comply on p.41) of
https://www.fairtrading.nsw.gov.au/pdfs/About_us/Publications/ft045.pdf
step back from the emotion – and consider how things look to a disinterested owner – plan things from that perspective – put everything on record so happy days can return – good luck ! 