Can you sketch a bit about the other arguments? Are they about the broader, general way your family operates, or specifics eg baptising a baby/ sending child to RC/CofE school type of thing?
Arguments about religion in a child’s life are pretty normal, if infuriating, but they usually subside pretty quickly when a parental fait accompli is presented.
However, beyond that you're entering much more extreme intrusion, and it should be resisted with force immediately.
You say in your previous post that (regarding the second birthday present) you aren't fussed either way, but I think you need to get your armour on and prepare to fight. Be emphatic, and take a stand!
Your in laws have no right to be such a heavy presence in what you and your husband are doing or planning. (Or any presence at all, come to that)
You say you don’t understand it, that it creates an atmosphere, and so on.
Well speak up, and support your husband when he’s trying to fend off MIL!
You gave in to her before your marriage on matters you haven’t detailed, and maybe MIL has ever since regarded you as ‘dealt with’ (and with good reason)
If you presented a more united front to her interference, she might withdraw.
As it is, she’s reading it all very differently, perhaps thinking she has the extra power and entitlement because she counts you as a silent ally.
Which you are, in practice, when you do not oppose her.
Until you find your voice and ensure MIL is faced with a solid block consisting of both you and DH, she’s going to regard your marriage and household as wide open to her influence, and carry on barging in with diktats.
She is relishing bossing her son about for myriad reasons, but you have the power to stop it, if only you would exert it.
Stand up for him, and to her. If it comes to it, decide with him that contact must needs be limited.
But for God’s sake, you and he have to do whatever it takes to ram home to MIL that you two are the only unit, the sole entity which decides about your children in every matter, not she. So she can rage and rant all she likes but her insistence will carry no weight, thank you very much.
Talk to DH and tell him you will not only stand by him in the face of her intrusion from now on, but you’ll endeavour to even do it yourself.
If necessary remind yourself that she, like any grandparent, has no rights in your little family. (Well, we exclude very extreme circs)
Assuming you have not withheld info about childminding, or any other dependencies on MIL, I think a course of shock treatment when she attempts her usual force is a good prescription.