Is money earned from treating women permissible or impermissible for a male doctor who does not know whether the woman has sought a female doctor and could not find one, and is it a condition to ask them about that? And does the ruling differ if the woman comes without a mahram?

There is no blame on the physician for his earnings, and he is not required to refrain from treating women if they do not have a mahram (definition) with them. However, he must not be in seclusion with them. Instead, there should be someone with him who prevents the seclusion. He is not obligated to inquire whether they have found a female physician or not. He should, however, advise women to go to female physicians.