Help with water changes, ATO and patience :)

[Hustler]

Ok so i have a 120 gallon water reservoir with my 100gpd 5 stage ro/di unit from aquasafe all rigged up for her first run... It has a check valve that senses when the water hits the in tube but i dont trust it and would like to install a float valve if they make them to stop the unit from pouring over?
Im very happy with how this is set up in my laundry room and will be adding a sink to the mix just to keep the washroom free of random fish stuff
Im so used to water on demand that waiting for 100 gallons of ro water is a real pain....I need 150G to leak test my plumbing and get my sump working asap.
Can i use my RO tank to mix and keep backup salt water or should I have a seperate tank for this aswell? I have a heater and powerhead in there now and Ill be pumping water from this to either buckets or my tank direct with a python and a 2500gph pump.

I should run the salt for a minimum of 1 day before i add it to my tank correct? cant just mix it per garbage bin infront of the tank for an hour and test with a refratometer?
Then.... For my auto top off system I hear bad things about trying to direct connect my ro unit to my sump so Ill have to find a decent looking container or tall tank with a pump for the top off, Anyone have any good plans or ideas for this? also what do you guys recomend for a float valve or controller for the top off system? It will go in the return section of my sump and shouldnt have any crawlers on it (i hope). My 300 gallon looses a ton of water each couple days so Ill need something bigger..... or find a better lid.
Any insight into any of this is GREATLY appreciated as Im fresh to the salt hobby and I dont want to start making mistakes now