1. Using a dish like you do helps some. But shrimp are messy and it'll still get everywhere. Just use tweezers to remove what you can, turkey baster or pipette/dropper to remove the rest. Even a syringe or medicine doser will work.
2.Yes. It's not that shrimp "don't like" something it's that their bodies can't tolerate it. Small water changes are fine. Or if parameters are spot on in the tank and the new water - even temperature - larger changes are fine. Just do them slowly and over an hour or two.
3. You can. But it's not necessary. I wouldn't.
4. Probably not unless you know what's harming them.
5. They won't starve. Only feed what your shrimp can eat in about 20-30min. Remove the rest. I feed tiny amounts for tanks with huge populations. You'll know they're hungry when they swarm your food. If they don't? They're not hungry.
6. Stop using additives. Lots (most, in my opinion) of them are a waste of money. Just flush the money down the toilet and save yourself the hassle. Seriously. When shrimp run off with food, keep an eye on it. Pick up what you can. Add snails to the take to help with cleanup. When it comes to feeding babies? Don't. Just don't. And that's coming from me - someone who makes and sells shrimp food. You should only be feeding babies when you have large populations and know you need something extra for them - or when you're a super-experienced hobbyist and know it's a good idea. If you aren't sure? Don't use it. I consider myself extremely experienced and only use my baby food occasionally - and an amount so small it's laughable.
7. Mites? Show us a photo. They're probably just daphnia and are totally fine.
Additionally - don't vacuum your tank. Only remove detritus that looks terrible. No need to disturb your substrate.
Just to be safe... What are your specific water parameters? Water temperature, ammonia, nitrite, nitrate, kH, gH, TDS. pH won't matter if you can provide the rest. What's the volume of your tank?
To make a short story long, a few months ago I set up a shrimp tank. For the first 3 months or so everything went well. Using remineralized RO water with the "right" parameters, etc etc. One of the things I did (probably wrong in retrospect) was that I just threw in some shrimp food on an unplanted section of the substrate and let the shrimp pick at it. Anything that wasn't eaten by the next day got removed. Over the next few days, shrimp would randomly pick at that area of the substrate to find food crumbs. Haha, how cute. Then every 1-2 weeks I did a water change and gravel-vacced that area, removing a lot of crap.
Sometime around the 3-4 month mark, the neo shrimp starting going inert, although the Taiwan bees were still fine, and I never really figured out the reason why. Around the 5 month mark, the Taiwan bees had babies, and so I started dosing extra Bacter AE and Shrimp Baby. Then things went downhill quickly from there, where the adult shrimp and babies got inert, babies died off after two weeks, then adults starting dying. Nitrates starting creeping up past 10 ppm, and I had a break-out of these little aquatic mites, or whatever they're called. There appeared to be little brown chunks of debris over the substrate and plants. Water parameters like ammonia, nitrite, nitrate, ph, gh, kh, TDS were all reasonable, tho. After a bunch of internet searching and getting advice from people, the only thing I could come up with was food-related water pollution.
So with advice from various people, I did the following things:
1. Put an intake filter sponge on, although it is not that big. I guess this is for extra biological filtration.
2. Occasionally dose H2O2 with the pump off. I guess the point is to zap bad bacteria and "oxidize" various pollution products?
3. Increase frequency of water change to 15% every few days
4. Cut way back on the feeding and use a dish and remove food after 1-2 hrs.
With this new routine, some of the shrimp sprang back to life, and some never recovered and still died after another week [grumble]. I still have the problem with the little aquatic mites, there is still some of that brown debris stuck in the plants (although it seems to be slowly decreasing), and nitrates have fallen back down to 2-3 ppm.
OK, now a bunch of questions:
1. What's a good way to quickly clean up food-related pollution?
2. Would doing a big water change be bad? I get that "shrimp don't like change", etc, but I slowly drip back in parameter-matched water over the course of several hours. If there was something wrong with the water, why not get rid of it as soon as possible?
3. Would it help to dose any of those bacteria that claim to eat waste, like Seachem Pristine, Dr Tim's Waste away?
4. Is it possible to save shrimp that have gone inert and would likely die in a few days? For example, put in quarantine with fresh water and give them some food.
5. If I cut way back on the feeding, how can I tell if the shrimp are starving? I know, biofilm, etc etc, but currently I don't have any more algae on the rocks (either due to low nitrates, or the mites ate them all)
6. How to prevent future tank pollution? The problem is that even if I feed with the dish, the Amanos like to grab a chunk of food, run off, and then drop it.
7. Still need to nuke the mites, somehow...