You can change the lock to anything if you have the Master Key for the currently installed lock.
Owner of a personal claim and villagers of a village claim do NOT gain visitor buff when entering through visitor gate located within aforementioned claims.
Not sure about the third question but it would make sense you gain two different visitor buffs, one for pclaim and one for vclaim. Visitor Buff will persist as long as you are within the claim and will remove whenever you exit claimed area, thus it's extremely important that your claim covers everything inside your walls.