Some questions and thoughts (major spoilers)

1) Seems like there are a few factors here.
a) they showed a scene where they were still working on it, it was recently finished
b) they weren't sure it would work...
c) you are trapped down there, so you cannot escape when one does get in... and one did..

2) Well fish are quite in the water and would be masked by rapids. I assume you are referring to the animals living in their house (I think they were raccoons?). Whatever it was it was probably a very quite animal that does hibernate in the winter... and just got lucky over 6 months? This may also be explained that for the first year or so of this alien attack the aliens may have been going after louder sounds, when there were more humans and larger animals. Because there were fewer humans now and less sound, smaller sounding animals would now be at higher risk.

3)
a) The shotgun would be a death sentence. As we saw in the final scene as soon as it is fired...they will swarm.
b) The creatures were armored and in most cases the shotgun I believe would have been ineffective. The hearing aid seems to have made their vitals vulnerable for a shotgun blast.

4) Indeed the Mom shows regret later in the movie that the child was not being carried. But also, the parents were preparing the kids to survive on their own. And really the kid is not much safer at home than they are out their in the open world. Really, anywhere in the world you make a sound it was going to end fatally. So what do you think is better, keep a kid locked up and bored all day(boredom leads to bad decisions)... or take them out and teach/coach them to live in this new world.

5) Military was likely over-run. There were newspaper reports meaning that the invasion/attack did take at least days? weeks? maybe even months? However we do not know really how many of these creatures there were and how quickly things escalated : sure there may have only been a few dozen in this rural area... but higher populated areas may have been hit with much larger numbers.
We saw a creature tear through a grain silo with ease, its possible that it wouldn't have much trouble going through modern military vehicles.

6) The only time we saw a shotgun be effective was when the creature was being hit with the high frequency sound. There is a note on the wall "armoured?". In a normal state these creatures are likely not vulnerable to shotguns or even heat seeking missiles?

7) When you bring people together you multiply the risk of conflict and/or noise. The larger the community, the more likely there will be noise and result in everyone's demise. These scenes with the light were really symbolic of people wanting to be together and forming communities but knowing that it would also be far to dangerous. A small hope of light.

8) The creatures destroyed anything that made sound, so if you turned on the fire siren it would only last 10 seconds.

9) I guess thats why they were making the sound proof room

10) so your question #10b question answers your question #10a: why the parents wouldn't let the kid have the shuttle without the batteries answers your first question. The daughter didn't see any danger of the space shuttle without batteries, but the parents understood that children are curious and the boy may someday find batteries and want to put them into the shuttle(too risky)... this happened rather quickly. Indeed, going back to your question 4.. you would expect one parent to lead and one to follow to prevent this kind of stuff... but I guess we can chalk that up to bad decisions be people under duress.

11) no ideas... yet!

Another question:

12) Where did all the water come from that flooded the soundproof room? Did I miss something?

1) No one was sure if the supplies they had for soundproofing would work.
2) Animals: I didn’t see any dogs, cats or cows around anywhere, only the wild animals that came out of the forest around the farm.
3) The father would have known what farm families were still around and felt reasonably safe, these trips were teaching experiments and dad was trying to empower the children. A shotgun would only work on these creatures when the hearing aid frequency was working, it looked like this messed with their hearing and slowed them down. Normally they moved too rapidly to even get an aim on them.
4) Can’t answer this one! The parents obviously haven’t read any David Paulides Missing411 books.
5) It was obvious,the creatures were thinking hybrids, mechanical with exceptional speed but that huge ear in their upper torso that moved like an old woofer speaker was human in origin. These creatures were designed to wipe out mammals. They came from the sky in an invasion made from superior technology.
6) You could barely follow these creatures with the human eye, not alone get a bullseye on one.
7) The operating survival strategy here was to NOT draw attention to your family nest or den. You may want to ytube “shark frenzy”.
8) See answer #7
9) It is difficult to build your doomsday bunker while trying to be silent. They were tackling one problem at a time and making sure the baby would stay alive was their priority for the moment.
10) She was just still a little girl who couldn’t hear.
11) The father had previously gave the daughter another hearing aid that proved unsuccessful and the girl feeling guilt about her brother was not in the mood to try anymore. The father didn’t want her to see his work on the aids or the cameras he used to watch for the safety of his family. He wanted the kids to live as normal as possible and be empowered.

13) What was the purpose of the creatures' kills? They obviously weren't eating what they were killing. Was it to stop the noise? If so, why would they be attracted to sound?