Verified answer

The contract to buy wouldn't be enforceable without both owners signing it but if they do both sign it then the contract with the realtor is also enforceable even though only one owner signed it. If the other owner doesn't want the realtor to be paid they'd have to refuse to sign the contract with the buyer too (ie not sell the property). As a realtor, however, you really need to get both owners to sign the listing agreement or you're asking for trouble.

nope!

Yes. The same way a lien can be placed on property held jointly, the realtor can enforce a listing contract on the owner who signed it to pay the commission contracted.

offer would have to be signed by both to be accepted

Yes. An owner listed it. If you are the other owner and you don't want to sell, then buy out the other owner. If you can't do that, then you take your chances with a stranger. If you can't see it working, consult a lawyer, who will know what to do about it.

Which contract are you referring to? The one between buyer and seller or the one between seller and agent? Did the co-owner sign the purchase and sale agreement? Your question is vague. Perhaps you should give more details.