On the water v starsan issue, as I use King kegs I’d have to make up 6-7 gallons of starsan to fill my barrels to sanitise and then purge. I’d then have to have a 7 gallon container/s to store the starsan for future use. I consider this a PITA so went down the route of using just a gallon (5 litres) of starsan to sanitise including a lot of shaking and swirling. That gets poured out and stored (leaving the foam in the barrel) and the barrel is filled with fresh cold tap water. My view rightly or wrongly is the water that comes out of my kitchen tap is clean and safe as it comes out of the tap so getting it in a barrel and sealing it with no or very few air bubbles has got to be very low risk. Pushing (or in my case sucking) the water out PDQ and replacing it with fermentation gas (CO2) must reduce the risk even more. Then doing a closed transfer from fermenter to keg immediately has got to reduce the risks to an acceptable level.
I think to flush the air from an empty keg would require high pressure supply of gas to mix with air and then exhaust. Sorry, I’m not convinced with a “slow” trickle flush