There's no locks per say, you either have your bank account linked or not. However, if you remove your bank account, your PayPal account is no longer verified which in turn will limit what you can do with your PayPal account.
As for how PayPal uses your funding sources:
PayPal will first look at your PayPal Account balance, if you have insufficient funds, PayPal will then take the funds from your Bank Account. PayPal will do an instant transfer from your bank account assuming you have a current linked and confirmed credit card. If the card is not current or no card, then the transfer from your bank account to pay for the purchase can take a few days, possibly up to 4.
If you have a credit card and no bank account, then the credit card will be used. This all happens assuming you don't elect to choose a funding source. Note, If you have both a bank account and credit card, you can choose to pay with your credit card on the PayPal Checkout screens by using the pay with another source option. There you can select your card from the dropdown.
It is possible to pay using the funds in your Account balance and the remainder with a credit card however, you must first elect to pay with your credit card. Your available balance will be used up and the remainder is charged to your card. As this now looks like you've sent a single payment, you remain eligible for Buyer Protection. You would not be eligible for Buyer Protection, if you paid for the item with two separate transactions.
What you cannot do is split a single payment between 2 bank accounts, 2 credit/debit card accounts or a bank account and a credit/debit card account. In addition, you cannot split a single payment between 2 PayPal accounts.
