I wonder if part of the issue is the ages at which we meet these folks. I think the challenge can be transitioning from something more formal to something more casual.
Our dear friends were high school sweethearts. He called her parents Mr. and Mrs. Last Name when they were in high school and college. When they got married they wanted him to call them by their first names, but he just couldn’t do it and was hugely relieved to be able to refer to them by their grandparent names after their first child was born.
My ds did not have any girlfriends in his high school and college days. If he had, I would have expected, “Mrs. Last Name,” to have been used. But I wouldn’t expect it now. Though, I would not mind it either. I think much has to with the customs from one’s own background and upbringing.
When I started working in my first professional job after college, I initially called the senior managers and partners Mr. Last Name because they were older/superiors/bosses (side note - in 1986, there were no female senior managers or partners). That was quickly put to a stop because everyone in the office called everyone by their first names. I didn’t start dating dh until I had been working a year or two, so when I met his parents it did not feel unnatural at that point to call older adults by their first names.
But, I think a lot of it is also habit. I cannot call any of my childhood/high school friends’ parents (now in their 80’s/90’s) by their first names - even though every single one of them has asked me to. It just seems disrespectful to me. I freely admit that when ds’s high school friends call me by my first name, I am somewhat taken aback because in high school, they called me Mrs. Last Name. The savvier ones now call me, “Queen First Name.” (See the current grandparent name thread 
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I have a very special relationship with a young woman who lost her mother when she was a teenager. She calls me, “Mama Last Initial.” It’s perfect because she is the only one who uses that moniker with me. It embodies the specialness of our relationship while still acknowledging that I am not taking the place of her mother. I love it.
I find it fascinating that people go years and years not referring to their in-laws as anything. But I know it happens. My friend’s dil is in that camp. I asked her what she does if she needs the salt passed, and she replied that the dil says, “Sam, please tell your mom to pass the salt.”
I do like the idea of having a joint conversation about it and deciding on something together if it is seemingly an issue. I realize that could be awkward but putting it out there seems far less awkward than going years and years with no way to address the person.
I wouldn’t want anyone but my ds calling me mom, and I would not have wanted to have called my mil that. My parents were both killed after dh and I were engaged but before we were married. One of the first things my to-be mother-in-law said when they came in from out of town to attend their funerals the next day was, “You can call me, “Mom.’” While I know her intent was good, it was just NOT the time to say that to me.
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I have no clue what the story is on that one.
