My therapist hammered into me for years the thought that what matters is what I think about myself, not what anyone else thinks. I still find it hard, but it's a good thing to aim for.
The above stands out for me, of all the really good feedback you got, Kay.
This won't be your mom's first grandchild. It's not as if you'd be denying her the excitement of the experience of her first one, you know? My MIL told me one time - after the first couple, the novelty wears off -- even though you're always pleased about a new person joining the family and getting to know them. Let's modify that a bit: lets think about it this way -
Your mom-skills are already top-notch. You're a fluent "mommers". You do have the challenge of fitting a newborn into the family and juggling attention for all 3 -- so that they all have their special time with mom. That is MORE than enough to handle - and we know you have other obligations, too. SURELY, as a mother, your mother can understand that you need your space and some time to adjust to being a bigger family. That's simply par for the course - it's not at all part of your relationship issues with mom. Those issues can wait for ultimate decisions and working through, if that's the path you all take. Because she probably doesn't see it that -- it will have to be enforced by everyone in a position to do so, not just you.
I'll even go so far as to suggest that, in mothering this baby -- in peace -- you'll also do a lot healing yourself. Nurturing yourself is no different than nurturing that little one. Those mom-issues will look a lot different to you after a few months of this. Save any decision-making till then - or even after. Just tune everything else out, let someone else handle it, your task is to take care of yourself & the little cherub. Mom can take care of herself - that's certainly not your job. Don't give it a single thought. That stuff isn't going anywhere -- but life allows you a good long "time out" from even thinking about it. It absolutely does NOT matter what anyone else thinks about this. Period. End of discussion. It's your perogative. And it's necessary, too - for the cherub and brothers & sisters. Mama Tiger knows she can defend them - so doesn't have to worry over that - and turns her attention to caring for the kittens/cubs... being there for them.
You are also well within reason to request that you aren't feeling up to the usual preparations and shindig of the holidays - so won't be hosting or travelling. It's normal to want to just stay home with the newborn and nest awhile... pick up those infant rhythms again and integrate them with the older children and the family schedules. It's even fun, we've found, to scale back on all that holiday hoopla... and enjoy the simple traditions of the season: making cookies, decorating the tree, and making simple, fun, secret presents and just enjoying each other - your very own family unit. It's a bonding ritual that will help create a life-long appreciation of the holidays -- without the commercial glitz, pressure/expectations, and stress -- it creates a happy memory. It also keeps the "melt-downs" to a minimum... especially with kids. They're helping to "make" Christmas... and not so focused on "what they'll get".
People who don't get that? Have an opinion? Well, who are THEY to judge someone else?
You know what they say about opinions: they're like buttholes - everyone has one.
It's not your job to take care of other people's happiness: except for your own and your family's. End of job description... no "other duties as assigned" clause that you are required to allow time and energy for.
I'd just love to be able to peek in on you & baby... and feel that transfer of mama-love in the air! What we didn't have -- we sure know how to create ourselves, don't we? We become the mom we didn't have - because we know SO well what it should be like. Or at least, we try!

Other things just aren't as important.