However, Cealee, you say that you are concerned that, if you 'give in' to this, she will carry on and complain about other, normal noises. You could find that, if you do 'give in' on this, it will make her feel positive about you, which might make more complaints less likely rather than more likely. But if you carry on waking her at night, that WILL make her pissed off at you - and if she is pissed off, every single thing you do will annoy her - and may generate more complaints, not fewer.
What Wolef said, YY to all her post, but particularly the above. As I tried to say earlier, as your baby gets bigger, you may need more not less understanding from her.
Your DH cannot be "against" FF if he is not the one lactating or struggling to latch a baby. I had a friend who had a 'D'H like this too, right down to the same details. The relationship didn't last due to his selfishness.
I have a preference for breastfeeding. I was lucky I didn't need to challenge my preference. But I would have mixed fed or FF in your situation, as there's no medical reason for your baby not to have formula. And my DH would have supported me to do what I felt was best for the baby and FOR ME. FF is not a big deal, it really isn't, whereas sleep deprivation really IS (as your DH seems to agree, as he needs to sleep through with earplugs, and as you seem to limitedly agree, as you let him sleep through with ear plugs but don't worry about your neighbour's sleep.) But apparently in this your sleep deprivation doesn't matter to HIM. If you weren't having to be up at 3am to express you might find things easier especially with the colicy baby.
Given your last post, about your preference to switch to mix feeding and your DH's attitude, I would urge you to consider how you have things set up. It is normal in some ways for the mother on maternity leave to do a lot of the nights, but don't set a precedent you will struggle to escape when you need to go back to work. By that time, if baby is not used to anyone else in the night, you'll end up doing it all AND working. You say you walk around with the baby in a sling when he cries int he evenings - your DH needs to be doing this as well - is he? It's really easy to become the "expert" and martyr yourself - resist it at all costs.
OP. You have had a pasting, because you are sounding like you can't see that it's your and your DH's problem to solve, not your neighbours. I hope your compromises work, but if I were you I would still just bloody go upstairs at 3am.