I don't think it matters what diagnosis she does or doesn't have.
It sounds like OP has accepted there is more behind her behaviour than simple thoughtlessness or disliking her/ not caring about the baby. That's the main thing.
Let's not worry too much about 'i don't recognise these as ADHD symptoms, you were right. It isn't a formal diagnosis. It's just her personality. She's just unkind'.
I am diagnosed myself with ADHD; and for the record do not identify with MIL's behaviours either but that's neither here nor there. just to say I'm familiar with the condition.
It sounds very complex re her childhood.
Things have only really just started to change around understanding ND and MH issues that may affect people's behaviour.
The hoarding would indicate some definite issues around material goods and security which I am not surprised are reflected in gift giving.
OP you've reached some understanding and recognised you're not the problem.
That's great. Work with that.
To be honest, the fien details aren't that important. There could be a thoughtless streak alongside some more complex factors. That's fine. Nobody is perfect. Develop your boundaries and stick to them. If she says something hurtful, it is fine to point it out. If you'd like to limit contact, do it (not saying go NC, just don't feel you have to go on every visit with DH).
And probably allow yourself a sense of humour and generosity around presents too. If she wants practical stuff, get her practical stuff. When she's gone, imagine the funny memories your kids will have of excitedly opening parcels to find shoe polish or soap. They honestly won't care they didn't get silver christening bracelets off her.
To add, people have very different ideas of meaningful, useful or desirable gifts. That isn't ADHD (Although washing powder is a bit extreme!) I have a much loved friend who sends me the most meaningful keepsakes, some with little slogans from craft shops and Etsy. Little pictures, pillows, ornaments, scarves, jewellery. Frankly, I would do anything for her and appreciate the thought but it is expensive tat that isn't to my taste. I would prefer something useful!