Reading your opening post I kept thinking, what a great job you are doing and that you had all the right information right there in your post!

I don't want to fill her tummy up with bulky stuff when BM is the best nutrition for her now
she's been going to bed at 7pm and going right through to 6am
DD had been fussing while she fed as she was trying to do a poo.
I asked her what reasoning she had and she couldn't give any.
I feel confident in my decisions, and the reasoning behind them
- ultimately this is the perfect reason to keep it up!!

I am so sorry she has caused you to doubt yourself as you really seem happy and on the right track. Fear and ignorance can be very powerful and negative reasons people will give you such negative feedback, so remember that that is all it is from your MIL.

As for the smile and nod approach, you certainly can do this as in the end her opinion is just that, but keep in mind that if you do this, you could be setting yourself up for more criticism of your parenting ideas later down the track. It may not be worth the energy at the moment or you may find that in these early days, you are in the perfect position to lay it on the line now. We are doing what we have decided is best for our child. We are well researched, and believe strongly in these decisions. We appreciate your opinion based on your experiences, but if you would like to read more about what we are doing so you can catch up to our philosophies, we can direct you to some fabulous resources!!

FWIW, I went BLS also, (Probably obvious given my response here ) and I did have the naysayers. My DD developed excellent eating habits, amazing fine motor skills early on, was handling spoons and forks like a pro way early, managed her own food intake incredibly well, learnt to chew and masticate a lot earlier than other babies, was never and has never been overly fussy with food, really seemed to love the independence her BLS offered her, never got frustrated and really embraced the ABA mantra - food is for fun, until they are one!

All the best, BLS really is a fun journey. I am sorry you are doing it on your own, make sure you join the BLS thread here so you can share stories with other mothers who are on the same journey. And keep up the good work!!!

ETA: WRT the MCH, mine was not supportive . I found it so infuriating that she was just parroting the official line and had not appeared to have read or understood anything about BLS. I just confirmed my approach with more reading on the subject and by watching my DD thrive