That's the issue of having multiple accounts (btw there's no point on owning more than 1 mc account), if you get banned or muted in one of the accounts, and you are found in the server with another account, it counts as avoiding your punishment so all your accounts and IPs are banned, I'm afraid.
Regarding your other question, if you are banned with multiple accounts, you only need to purchase the unban fee in one account and it will be enough. Since all dates are saved, we will know that you purchased the unban fee after you got banned so you aren't bypassing any ban at all
Hope that clarifies things up.
Have a good day.