What might be causing rear casette to creak?
It sounds like you have isolated the issue to the rear wheel, and probably the rear hub.

I doubt that that cassette or it's attachment to the freehub body is the cause of the problem. It may be the freewheel assembly, or the hub bearings, or even a crack in the hub body.

To rule out the cassette, you could replicate the creaking using a chainwhip tool as show in your second video, then remove the cassette and see if you can replicate it by torquing the freehub body. It sounds like you don't have the tools to do this though. Do you think you could you work with a repair technician at the bike store and have them use a chain whip tool demonstrate the creak with the wheel out of the bike?

You say the problem has existed since you bought the bike. Were all the previous unsuccessful attempts to fix the problem performed for free by the store you bought the bike from? (In order to make good a faulty product that they sold you) If so, you can try requesting that they replace the entire rear wheel, or at the very least the entire hub. You may or may not have success depending on how long ago you bought the bike and warranty it came with.