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.

She’s being a dick but it seems VERY pointed to honour your family and not his…

That's interesting isn't it.
She has other grandchildren but hardly sees them.

Sounds like it's time for your DH to take over all of the communicating with his parents while you just enjoy your new baby.

Agree 💯 with kitty or not to kitty

It isn't surprising that she (or rather FIL) might have felt a little bruised to see your Dad and Grandad's names used - 2 from your family.

But it is her job to keep schtumm, say 'what lovely names' and get on with being a happy grandma and loving the child whatever his name.

I would say "I know I was snappy the other day - I just want you to know that we just love that style / era of names, and those names in particular. It wasn't a way to make one half of our baby's family more important than the other. " and if she comes back on it "well that's his name now. I am sorry you feel upset but there's no point going over it - the most important thing is that he is a happy, healthy, loved child, so lets focus on that". And after that ..."we've had that conversation, ooh look its raining"

And take control of the boundaries, Give her a job. "We nap / rest / in the afternoon and are in bed by * but it would be brilliant if you could come on Thursday at 11 and wheel baby out in his pram for half an hour - see you then"

She sounds difficult, and a pain, but I wouldn't burn bridges yet.

You and h get to choose your kids names.
Mil and anyone else sticking their oar in need shutting down pronto.
Good for you OP, fuck being kind and nice sometimes. You told her, your h was on side as he should be. She has to get over it, think all the more but get over it.
I know someone who named their baby, in the week after a dodgy A lister was accused of serious sexual offences. Their kid wasn't named after this person but given the same name. I was 'oh, that's nice' while thinking wtf.
The news has moved on but the name will still resonate with people for the wrong reasons.

YANBU.

Anyone who repeatedly turns up at the home of a post-partum mother to whinge and harangue after getting a reasonable explanation clearly needs a more direct conversation.

You’re not wrong - she had her chance to name multiple children. She doesn’t get to name yours. As long as you and your husband agree then it’s nobody else’s decision.

I cannot comprehend taking the joyous occasion of the arrival of a new grandchild and turning it into something to be furious about.

A bit dramatic.

Naming a child is usually decided by the child's parents.
It doesn't require committee approval.

My own - generally lovely - Mum said far far worse when I told her my children's names. Negative input is by no means restricted to MILs.

I’m a MIL. I wouldn’t comment. Their child, their choice.

I say well done you. She pushed and you pushed back. The dear child is named and that’s it. Start as you mean to go forward with MIL. If she’s pushy this won’t be the last thing she wants a say on. YOU are the mother the wife and it’s your home.

She needed telling.

However despite you and DH both agreeing I can imagine it’s hard for his mum to have the GC born to him being named maternal father maternal grandfather. No acknowledgement of his family on the name.

Not saying you’ve done anything wrong but I think we are quite often conditioned now to look at peoples behaviour without considering their motives and reasons - which need to be acknowledged to move forward.

This.

Clumsy of you to honour only one side of the family!

But also yours, so not really.
I'm not saying MIL is right, but 2 names from your side of the family and also your surname. She may be feeling a little hurt, like husbands family are less important. Which is valid but doesn't really give her the right to express it either.

‘Contaminate’ is very over the top. Although the rest of your points are valid, you loose moral high ground by talking about people like that. She’s over stepping but she just loves her family, she’s not a disease

Obviously totally your choice what you name your child but I can see it feeling a bit one-sided having both names from your family. That’s likely what she’s really upset about but doesn’t want to say and that’s why she’s criticising the name.

She’s definitely rude to make comments and bring it up all the time. But you were rude to snap and not just bite your tongue until she left. I get it though, postpartum is a tricky time.

You could both be doing more to try and preserve family relations long term. If you care about that… which maybe you don’t if you don’t want her hanging around!

Why would their opinions and feelings about the naming of their grandchild be relevant?

I firmly believe that if the parent of an adult has a problem she or he should raise it with their side of the couple.
I had MiL disagreeing with a decision we made about nursery in COVID and she sent me link after link to articles "proving us wrong". I forwarded them all to DH to deal with.

You can name your son whatever you want and she can be upset by the names you have chosen.

Naming your child after other people always going to end up with someone being upset.

I understand her feelings, you're honouring your family and not your husbands. To me this implies that you think your family is more important than his.

I think Op has sent a very clear message to her in laws that their opinions and feelings are irrelevant and will not be considered in the future.

Don't understand why she needed to go about it . Did she think if she complained that you would suddenly decide to change it to suit her . I do find it hard to understand her motive , voice her disapproval and alienate you ? You will never forget that she did this. But in future , let your DH deal with his people .

It’s your and your husbands decision and no one else’s. My ex MIL was the same. We named the child after her own husband (middle name) and gave him their family name. Me or my family completely unrepresented. She kicked off because her side of family weren’t included and wanted quote ‘her daddy’s name’ to also be included. She said ‘you need to include both sides of the family’ as in her and her husband’s. My ex said nothing

I mean telling her to have one of her own isn’t great is it. I can see why she was really hurt by the name choices, but she shouldn’t be telling you that repeatedly.

I assume that your MIL was able to choose your DH's name? She does sound like a pain in the arse. Tell her that the more she behaves like this, the less time she will be able to spend with the baby because she is just upsetting you on purpose. If she can't be supportive, she won't be welcome in your home.