I am by no means a tuner for BMW cars, but i've tuned several cars in the past myself, and am using BM3 at the moment for the BMW while I learn the new platform. That said, heres my take.
1 - E can be from 50 at a minimum to anything more. for example the SHEETZ gas stations in my state routinely have E90 during the summer months and then E70 for their winter blend. So always test at the pump. I will typically not test the gas as I've only come across 1 station so far in so many years that is not e10, But yes youd use the e content to do the math to figure out how much of each you need to obtain e30 into whatever final quantity you desire.
yes it's ok to be off, just like it's fine to have crappy gas. but you want to set yourself up for success. I typically shoot for E33 and round up to make to the nearest half gallon to make it easier. just like adding a couple gallons of E to normal gas to get mild improvement on a standard 93 octane tune, the slightly higher e30+ content will accommodate for other issues.... but don't go too high.... no need to be exact, just close.
2 - again do the math. I use an app called FlexCalc to help. but in general if I am going outside my normal driving routine then I'll just run the tank down and flash back to 93. 93 is everywhere on this side of the US so it's a safer bet. when I have predictable conditions I go back to e30. But if you want to figure out exactly, just change your desired total end volume. its no longer 17g or 16g, its 8 or 4 or whatever, then calculate with the desired total volume to be that instead of a full tank to get the needed ethanol vs 93. gain flexcalc can help.
3 - If you are at e30 and you are low on fuel with no ethanol to be found, just flash back to 93 and fill up with standard premium. a bit of E in the tank is not going to hurt.
4 - again, read the above, think you are complicating it
For reference I've gone all in and am waiting for my two 55gallon drums to arrive so I can fill up a load of fuel to get me e33 blend on hand all the time while I'm home. should last me ~3 months with that quantity. so I'll fill up whenever I'm home at 1/4 tank or less, or more... whatever. my station is over an hour away and my 6x 6 gallon jugs were becoming a pain to refill so I was more 93 than I was e30.... this is my fix as I gain money for my dorch stage 2 pump