To tell my MIL if she wants a son to name she can go have one and not be engrossed on what I choose to name my son.

When my DD was born, we gave her DH's Mum & my aunt's name (same name) as a middle name. Then when DS was born, we gave DH's dad and my dad's name (again, conveniently same name, although the're known as different diminuatives). Since we weren't going to have another child, and we realised my poor Mum had been left out, we asked her to choose a 2nd middle name for DS, and she was delighted. It's a lovely little family story, with all the children's names having meaning and shows how as a family we love, consider and respect everyone. IMO, this a much nicer start to life than OP's 'my way or the highway' approach. OP is techninally right, she can name her baby what she wants (with DH's agreement), but it was a dick move that was obviously going to cause hurt to her child's paternal grandparents. Would it really have hurt to give him a second middle name from his Dad's side? It didn't have to be FILs name if that's really unpalatable to them. His own Dad's name/middle name or his Dad's grandad/brother/uncle's name/middle name could havdle been considered.

She’s being ridiculous. Our first is named after my maternal grandmother. Our second after dh’s dad. Neither of them were alive when our dc were born or we might have consulted with them about using their names. Otherwise, no one in either family knew our name choices. None of their bloody business really. I dare say this does not bode well for how she’s going to be as a grandparent in years to come though.

My kids don't have names that favour either side of the family... so we're not weird either.
My point was it seems she has entirely favoured her side, and left out husbands side completely.
Its an obvious 'oversight' snub that MiL has picked up on as was clearly intended

Is there something else going on in terms of your husband/MIL's culture?

In my family tradition the eldest son is named after w, the eldest daughter after x, the next son after y, the next daughter after z. Then if there are more children the parents pick the names. Sometimes the middle name is the mothers surname.

Obviously with smaller families we don't get to use all the old relatives names now.

There was also a local superstition that the names were reused so that the dead person was 'reborn' or 'at peace' (or something else along those lines).

For a family not to reuse a name was a big thing. When it happened it was a deliberate snub.

I have noticed that my age group were the last to be named on this way though. Most children are being given unique names in the family or ones based on the old names. They are using middle names in some cases from both families.

One member of the family has married someone from another culture where they think it's bad luck to name a children after a living relative.

Your MIL may have taken the name as a direct snub. It's up to your husband to explain this to her though.

Well I guess it would have been nice to have used a name from their side but if there's none you like the that's that.

My son has the same middle name as his dad & paternal GF.

If I didn't like it he wouldn't have it!

I don't understand why thought that they should be consulted!

Despite having had four of her own, my MIL (a lovely woman, and much loved by me) was keen to name our firstborn son and had one name in particular she really wanted us to use. Just to tease her, we started referring to the fetus by a very old-fashioned name indeed. Originally it was joke name, but by the time he came into the world, we were so used to calling him by that name that we made it official. Fortunately, he likes it!

Regardless of names no one should be coming every day to view the baby. You and DH can decline daily visits, you are settling into your rhythm as new parents, no more daily visits, thank you. We'll text next week for a catch up type of thing.

We named our daughter incorporating MIL name as a second name and she still was not happy and made a snide comment about the first name we had given her. She got used to it over time and loved it by the time she died. She was imperfect same as the rest of us but on balance I loved her a lot.

What do you mean by contaminate? Do you mean she visits with colds and coughs?

You chose names that honoured your family and ignored your husband's. In return you get the pleasure and appreciation of your folks and the disappointment of his. Bouquets and brick bats.
Your MiL should have controlled her disappointment certainly but you can't accept praise for the names from your family while considering the flip side of that totally unreasonable.

Is it damp under there?

Yes that is the line to take. Pick up on a 3 week post partum mother for momentarily using “my” instead of “our” when referring to the child she has carried for nine months and just birthed. 🙄

MiL is ridiculous and the only debate to have is how to limit her intrusive and unhelpful visits.

I think you are utterly selfish to have both names from your side of the family AND give him a ridiculous double barrelled surname.

HRTFT, but just remember this might be you one day, especially as DM of a son. I say that as someone who is grateful to get on incredibly well will my DIL, and whilst I will never be as close as DGD's maternal GP, I have a wonderful bond with DGD.

And remember, one day OP, you will be that paternal grandparent, a future MiL to your DS’s wife. (Or husband).

"My family are absolutely thrilled. My dad in particular is over the moon and feels incredibly honoured that his grandson has been named after him."

No thrill for them, would have been better wording.
All newborns are a delight.
Naming after her Dad, fair enough.
Husband not bothered about his, okay.
But making the middle name her granddad's is a snub.
As others have said, her baby, her choice but it is tone deaf.

My kids don’t have a nod to their paternal side. One has my dad’s name as their middle name. No one fought or argued over the names of their grandchildren. Our family isn’t weird like that.

I'm imagining a baby Cuthbert.

I wouldn't have picked both names from one side of the family but I only have only used one family name in 8 names.

It's none of her business and now she's been so rude any chance of you picking two names from your husband's side next time have gone.

Congratulations.

Are your sibling and his wife having a son?

my son

Another MN virgin birth? Surely he is 'our son' and you and his father will choose names.

Why isn't your husband telling her to back off and give you all some space while you nest? She needs to back off and he needs to be clear and make the boundary, not you. Next time she asks to come round HE needs to be the one to say sorry, no, we want to spend time together as a family. She is coming round daily now, this will turn into twice daily soon enough and turning up unannounced.

I have friends that didn't let anyone see the baby until they had their first jabs, she needs to back off!

Paternal grandparents often feel a bit less connected to the child and worry they’ll not be as close as the maternal line. You’re right with what you said but if you want to have more empathy for her look at it that way - she’s worried that she won’t get a look in and now the baby has been given both names from the maternal line. She just feels pushed away or left out.

I wouldn't have snapped at her, but I would tell her exactly what you did: that it's your son, you get to name him, and you don't need to consult her or anybody else about his name. End off.