I need help removing Activation Lock and do not have proof of purchase documentation, as iPad was given to me

As you are not the original purchaser, proof of purchase would be academic in any case. Without knowing the APple ID and password used to lock it, of the previos owner who set the lock removing it from his list of devices, there is absolutely no way of your unlocking the device. Apple won't do it: and the websites which offer to do it for a fee can't either - these are scams.

62-331 wrote:

How can I return it to its rightful owner?

You cannot.

And no one will identify that owner for you, as that’d potentially leave the owner open to harassment or worse.

The owner had the opportunity to post info to the device to ease the return, and chose not to.

Without proof of purchase, that iPad is a paperweight.

~Lyssa