How does "optymalizacja zus" companies work exactly, am i an employee in the uk for some company?
No, they just register you as self employed in the UK and charge you a nice whack of money for doing so. Still cheaper than being self employed in Poland, however. Also worth pointing out that no-one takes this kind of company seriously - if you're doing business with Poles, they will expect a Polish company.
Not true. I have used the EHIC card on several occasions - most recently in September for treatment for my daughter. No hassle whatsoever.
Very lucky. It worked for me in the past, too - but all it would take would be one administrator questioning it and you'd have issues. A clued up administrator in Poland would question your residency, especially if you couldn't prove that you were UK resident at the time. It's also worth pointing out that if the UK figured out that you were using it while resident abroad, they'd go after you for the amount paid - which could be considerable amounts of cash.