3. I suggest you only buy vaccinated chicks until the infected chickens (exposed, unvaccinated) that you have now have all passed - even if this is 10 years. After a good amount of time (I've heard 1-2 years) have passed without them in the flock, then you can think about hatching and consider yourself Mareks free (maybe).
Probably not quite.
The vaccinated ones can still get the disease, carry it, and spread it.
The vaccine just makes them less likely to show symptoms, and less likely to die.
As long as chickens are continually present, Mareks will also be present--regardless of whether those chickens are vaccinated