Olive Mac sounds like a 1930's jazz singer or something, I like it, and personally I'm not a big fan of the name Olive at all, so I think 'Mac,' improves it. I also love that youre using sentimental names rather than just fillers.
I don't call the raincoat a Mac/Mackintosh and don't know anyone who does, though I'm aware its a well known term, so id never think of that.
I gave my first born son the two most sentimental names we had as middle names, because who knew what the future held, I might not have conceived again or I might have had a girl so I don't agree with a PP about saving one of the names for an unknown further child.
Matilda Olive Mac 'Williams' is a great name.
One last thing, middle names are almost never heard or seen again by anyone but you and your child following the first few weeks of her life so you can really stick anything you like in there, it really really doesn't matter how they look or sound, but is a great opportunity to acknowledge and honour loved ones.