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To make an informed decision, "#3" would need access to at least one of the two involved Paypal accounts, right?

If it really comes to that, I will let #3 login into my Paypal account. If for whatever reasons this is not possible, consider me as a fraud and #3 should decide in favour of the seller.

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Chances are 1% of 500$ is not enough to pay the fee

Fees are payed by me. #3 receives 1% of the total involved BTC.

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"To grow confidence" is a very tricky statement

I mean growing confidence into this process. Like I'm not doing multisig everyday. I'm planing to keep this process and you should distrust me always.

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To take an informed decision, #3 needs to know the details of the deal, and keep track of it for up to 180 days. That's a lot of work for a maximum of 5$.

Details will be available per CC. And it is just "I buy X BTC for Y USD from seller". That's it. The thing is, I really expect that #3 won't be involved at all, because the main reason for #3 is protecting the seller against me being a troll for charging back and letting the BTC rot. So now with #3, I think it is very unlikely that I am really a troll and #3 might never be needed. So it is really 5$ for just providing that key and not loosing the private key. If #3 thinks 1% is too little, please make a counter offer.

Also I was looking for the chargeback period and didn't find anything useful yet. Do you have a link to that 180 days?

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Say #3 is not involved until the end. How will you provide evidence of the details of the deal?

#3 gets the original deal at the beginning. Evidence can be collected from my Paypal account. If I don't provide access consider me a fraud.

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You should be much more careful who to trust.

That's a good point. I really have no idea what criteria would be sufficient. What would you consider trustful?