I make Instant Ocean saltwater in 200gal batches and store it until I need it. The longest has been about 6 months. If you have a small low demand system you really need to match your water parameters (alk tends to drop over time). If you are doing a change on a high demand system the amount of supplement in the water is minimal so storing the water longer isn't an issue at all.
1) a long time depending on salt (stuff with vitamins won't store)
2) Yes, a Reeflo Dart.
3) Unless there is a problem I don't break it down. I use muriatic acid, a garden hose hose, and a python siphon to clean it.
Fill the 1st tank, add a mild solution of ma, and let it sit. I then transfer it over to the salt tank and let it run until it's clean. Once clean I pump the water out of the tanks through the hose I use to deliver water to the tanks. I use a python to quickly remove hose water and rinse down each tank thoroughly. After I am confident they are clean I run some water through the whole system, drain the system, and then air dry.
If you had a normal sized setup it would probably be easier to break it down, but then you wouldn't get the crud in the pump and tubing.
I use boxes of salt which come with four 50gal bags. If you really want to make it easy on yourself get tanks that are in 50 gal increments.
It's probably overkill but I have a mixed bed DI canister running on a closed loop in my storage tank. I use an Eheim 1260 pump with plastic hardware. DI is corrosive. I refill it when it gets to 60 gallons or less.