I don't think you are allowed to borrow against a Roth IRA. You are able to borrow from an IRA in a very small set of situations. I think they are mainly buying your first house, going to college, becoming disabled, or adopting a child.

I am not a lawyer so take it with a grain of salt and I am very probably wrong.

Of course you are allowed to borrow against an IRA. You can pull they money out, at any time, day or night, for any purchase. You just have to pay tax on it.

There are some situations where the tax is waived

It's explicitly prohibited for Roth IRAs.

That's not allowed.

I expect if you have enough there, you could use the same sorts of schemes as other illiquid wealth.