Hi. It's a difficult time for you all.
i have been through it.
my advise is that you need to take the long term view, and that means doing what it required to maintain a calm situation for the sake of your daughter. She will no doubt love you both and be very troubled by what's happening.
sorry if that's a leature, but I was given the same advice and whilst it was dificult to do it, I am glad I did.
so..
re your firearms - When I was going through this, I was concerned my ex would make up a tale that I threatened her with them, just to make trouble, so I took the proactive descision to move them to a dealer (informed my police) and left them there until the house was mine and the locks change.
Admitedly some inconvienience, but I slept easy.
re the locks - I'd say don't change them until the house is yours.
I am not a solicitor, but I don't think there's any merit in claiming you need to change them early because of the firearms, because they are locked in a safe. And that's been ok whilst you were together.
As I say, it's a tricky time for you all, good luck.. M.