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As far as I've seen, Paypal can be charged back up to 180 days later.

Thanks, I updated my initial post.

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3. After 170 days, Bitcoin dropped 80%, and your 500$ in Bitcoin is worth only 100$. You don't want to take a 400$ loss, and charge back through Paypal. Seller complains, multisig #3 sends the funds back to seller, but seller lost 400$ anyway. NOT good.

Ok, that's a risk the seller has to take. Sorry, I don't know how I can protect the seller against that. The seller has to consider this risk and adjust their commission accordingly. If you have any ideas, I'm totally open for it.
Ha! I think I might have a solution to mitigate this risk. I would also send a deposit to our shared address. In case I chargeback, seller may keep my deposit. It might still be that the rate change is not covered with the deposit, but at least the risk is lowered a lot. I think with a deposit from my site, even a third signer is not necessarily required (for the chargeback case) as I also have a personal interest in getting my deposit back.

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I want to buy BTC with Paypal. ~ I plan a long term partnership to do recurrent deals in the region of 500$ a month.

Serious question: why? There are better and more trusted ways to buy Bitcoin.

I'll pm you, whenever my quota is restored.