Spektre did a good job answering your questions, but I'll add my 2 cents, too.
1. I like all the driving options. I typically use B around town (~35 mph or less) and D1 on the highway (no traffic) or D2 with traffic that still moves but may slow down occasionally. I almost never use D or D3, though I have used D3 driving down a mountain. D is the experience of driving a gas car, and not to my taste.
2. I rarely see 6 mi/kWh but getting over 5 is not hard in the fall and spring (and much of summer) in the SF Bay Area since driving locally, I will not have the A/C or heat turned on and almost always drive in Eco mode. If you do a lot of highway driving, you will not see over 5 mi/kWh but low 4's is possible.
3. Yes, the J1772 plug will always lock into the vehicle inlet, unless the plug has a broken locking lever, and then you need to be very sure that charging is not active when you remove the plug because VW did not properly implement the J1772 communication protocol and you'll get a spark if you pull out the plug while the car is still charging with a broken plug lock lever. Yes, you always need to unlock the car to get the plug to release. On the 2016, I think you have about 5 seconds to remove the plug before the lock re-engages. On the 2017 and later cars, there is a 30 second delay. Also, I have found at many public stations to get DCFC to work, I need to lock the car prior after inserting the DCFC plug. If only the e-Golf had Plug&Charge....
4. GOM is not influenced by either L1 or L2 charging, but is influenced by how you recently operated the car (heat on or off, high speed vs low speed, etc.).
5. There is no way to save drive mode or regen mode - I agree it's a pain, but I am used to it after driving an e-Golf for 6 years.
6. The detent you found at the bottom of the accelerator travel is there for those time you need "maximum warp". If you drive in Normal, then I don't believe the detent does anything, but if the car is in Eco or Eco+, pressing to the detent gives you the full power output of the motor. In my car, in Eco mode, the motor puts out 85 kW but 100 kW when the detent is pressed. Not sure about your car absolute numbers - you'll need to look it up.
7. Yes, setting battery SoC to 80% routinely will prolong the battery life. Charge to 100% for long trips. Press the Car button on the center display and then go into the e-Manager to create a charge profile where you can set maximum charge %.
2017 e-Golf LE, Multi Coat White with Black Vegan Leather interior