Honestly it sounds as if the Seller is a total jerk, and frankly if their already this bad why would you possibly want to buy their home. The entire transaction is going to be nothing but painful, If I were you and wanted the house (which I probably wouldn't after encountering the Seller) I go back to them and say the new price I'm now offering is $120,0-00. My deal would be as-is (trust me you don't want to try and deal with this idiot on home inspection issues and that my offer was good for 1 day only,.
If you want the best advice I an offer, it's forget the house, file a complaint against the seller /agent and move on. Buyers should remember there's always another home, I doubt there anything really remarkable about this property and life is too short to waste it on jerks.
How frustrating! Let's keep this simple and try to not let your emotions get in your way.
Sign a Buyers Agent contract and become their client. Have your fidicury buyers agent submit your offer with all your qualifications: mortgage pre-approval and anything else that demonstartes to the seller that you are very qualified to purchase their home.
If the MLS states that a commission will be paid to a buyers agent then if your offer is accepted your buyers agent should receive a commission.
Not to confuse you, but to help illustrate a point, there are times when an MLS will state the commission to the buyers agent is, say, 2% but your buyers agent contract states they get paid 2.5%. In this case you could have to pay your agent the .5%. It is true that if the MLS states 0 commission to a buyers agent then the buyers agent will describe how they will get paid if they expect you to pay them. Please make sure you read the buyers agent contract before signing it and discuss it with them.
Usually "procurring cause" of the accepted offer is what is looked at. Meaning, who actually brought a ready, willing, and able buyer's written offer to the seller. Notice, I said "written" offer.
The seller's agent cannot make decisions without the sellers ordering them to do so within the scope what is legal.
Your Buyers Agent will advise you from here on.
So, Take a deep breathe, relax and let a professional work for you.
If you did not sign a contract with the selling agent to have them be your agent, you have no legal obligation to them. If you want to have a different buyers agent, make sure you sign a contract with them. Have your buyers agent submit an offer to the listing agent.