16 minutes is correct.
15% calculates out close enough, it should be fine.
2 gallons above the grid... I dont have my chart handy, and I have not had to remember those numbers for at least 15 years since I stopped using the Erie 541. Depending on the type of grid, it may not matter. Does your grid have the leg holes? or it is a completely flat top grid? If it is the flat top grid, and the water does not reach the grid, you can simply remove the grid, or raise the bottom air check. I have my salt chart at my desk, I can answer it more accurately tomorrow. How high is the grid off the bottom? You can easily test it by filling water to above the grid, and simply run it through a manual regeneration, an hour and 30 minutes later your question will be answered.
The normal start up procedure is to add 5 gallons intitially, then regenerate. The system will automatically fill the brine tank after that to the proper amount.
I would use "funny pipe" by toro, it is a high quality semi rigid black tube that uses the same toro barb fittings. This small of a drin line will be fine considering your systems maximum drain rate is 2.0 GPM.
Only one person here who hates the 7000 continually comments on the size of the valve. As I have shown with picture proof, it is the same size as the 5600 and 5600SXT when the standard meter is used. But picture proof, technical drawings, etc, are all not good enough. Maybe your comment will help to educate the misinformed.
Congrats on your new system. It is a good one. By my calculations, it should regenerate every 8-10 days.