Welcome to the boards.

We've all been where you are, and most of the time it's a learning experience. When adding new fish to an established tank, you should have a quarantine tank to avoid passing disease to your established tank. Also, if you're doing weekly water changes, you only want to do 10% water changes as more than that would damage the bacteria.
I'm sure that the sudden change in pH may have increased your casualty rate because it's not just a little jump from like 6.0 to 6.5 it's a log scale, so that's a big change for healthy fish let alone stressed fish. The salt addition was a good idea though.

The huddling at the top of the tank indicates low oxygen levels, so you'll probably want to add an airstone or two especially since there's low oxygen in water in normal circumstances, and now they're stressed so they're using more oxygen. The milkiness of your tank may be an indicator that your ammonia is going to jump. Are you using an ammonia reducer?

The answers to your questions:
1) Patience is necessary as your water parameters won't return to normal overnight. Water changes (when ammonia is high) and highly oxygenated water will help your fish. If you don't notice the fish rubbing on things, you should be able to treat the ich and recover your fish. Like I said, check your water prior to putting it in the tank. If you're using tap water, you should let it sit for at least 24 hours to let out any toxins. If your pH is low from the water you're using you should adjust it prior to adding it to the tank, but be sure that your water in the tank is similar to avoid a massive jump in pH. However, if you fish were happy then, they shouldn't be sick from the pH. The fish you mentioned are pretty tolerable, and I think it was just the ammonia in the tank from overcrowding and possibly the ich came from one of the new fish. Stress will increase the likelihood of your fish getting sick, and the addition of a new fish starts your cycle over again. That's stressful for fish.
2) The water you are using could be the reason your pH is low. Have you tested the water prior to using it?
3)It seems you are starting all over again, but with frequent water changes (and lots of water being changed) you can upset the bacteria, and will need to start all over again.
4) There are several treatments available for ich, and I can't recommend one with the best recovery because the ones I used didn't work well (I lost all my fish), and usually it's too late to cure your fish. The good news - any of your fish that do make it through ich, will have a resistance to it. Also, your baby guppies may die when adding an ich treatment because they are so small and are more susceptible to illnesses (and therefore any chemical treatments in the tank).


Hopefully this will help, I realize it's been a couple of days, but I hope you can still use the information. Don't give up on your tank, mistakes happen, and that's what we're here for.

Keep us posted on your progress.

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