their wealth is not in a single wallet, but rather in 20k wallets. So instead transferring bitcoin, they could just hand out access those wallets.

The receiver will immediately move the bitcoin. So it has the same downside.

If the receiver doesn't immediately move the bitcoin, the receiver is at risk of Satoshi stealing them by retaining the private key and moving them later.

Even if the receiver trusts Satoshi, if the receiver wants to spend the bitcoin on anything, there's the same problem again.

Sure. So just a case of 'trust me bro' on both sides. What could possibly go wrong?

You still have to pay the interest from somewhere. And presumably you'd need to put the coins into some kind of escrow so that the lender can get their money back even if you conveniently forget your private key.

In this case it's not basic stuff. You would need to prove that you own the actual bitcoin or transfer it for it to be collateral on a loan. It's the same as spending it.

Why are you framing this as the only solution? There are many many other ways to secure a guaranty