You have a fundamental misunderstanding of how Bitcoin actually works. It might be better for you to start reading up on it before asking questions, so it'll be clearer for you as well.
The (PSBT) format is created in such a way that if the transaction is between three (3) persons, the two people who are received will have to create an address and add to The PSBT.
PSBT in the sense of a MultiSig is with an address that has a Multisig script hash, that address will have the specific requirements (ie. the signatures) required for a transaction to be valid. We don't add addresses to the PSBT, the PSBT contains the information needed for someone to generate a valid signature for the transaction that they've created.
1)At this point,the signature of all parties are involved.what is the rate of impossible when its comes to any participant who has evil motives like accessing anyone's signature?
No chance.
2) Are signatures created with perfect privacy conditions so therefore cannot be accesed by any kind of fraud?
If NO,then PSBT isn't SAFE!!!
3) I have read that sometimes,coordinator sends the PSBT to each of the user to sign them privately and Send it back,in these case,since bitcoin is decentralised and people aren't known specifically as any frauder might just decide to become a coordinator just to fraud everyone of their coin, don't you think and improvised method should be used on these maybe to the MINERS directly?
No. Privacy has nothing to do with this. You are supposed to authenticate and be sure of the people that you're doing a MultiSig with. If you don't you might be dealing with the wrong people and could possibly lead you to getting scammed. Whoever is signing their part are supposed to be aware of and know what they are signing for, we don't sign anything blindly.