I get the full battery charge (all green bars) every day since I commute over a mountain gap. Going downhill and not accelerating but braking and regen braking though the ICE (Internal Combustion Engine), the battery will fully charge within a minute or 2. At that point the ICE will spin (sounds like a revving sound)--very highly if in "B" mode and moderately in "D"--to prevent overcharging. So just go down a hill for a while and you'll charge it fully. There may be other ways it occurs that someone else can suggest.
As to fuel meter accuracy, it is pretty accurate within it's limited range of up to 99.9, although there's a 3 second delay. Parts of the following thread shed more light in more technical detail on its accuracy:
Most people won't see their battery levels at either extreme other than very rarely. Any time the battery hits one of the extremes, it's an indication that the battery is too small for that particular driver's behavior. Since the battery was sized to be sufficient for most drivers, they don't see the extremes often.
I live in a hilly area of San Diego County and will usually have the green battery in the early morning commute. It does not last long but it does appear fairly regularly. May you just don't catch it, afterall you do have to watch the road part of the way....
To answer the 2nd question, the MFD averages about 2 MPG higher than actual, accounting for tank fill variances by using multiple tanks.
Back to the 1st question, I get 7 green bars occasionally, especially if I drive long stretches with the AC off. But I only got 8 bars once I think.
Never drained to 1 or no bars. Not really possible without being out of gas or in neutral too long. I am too chicken to go to 0 bars in neutral, though I believe, and I believe someone has said, that the car will simply shut off or go to Ig-On if the battery SOC drops too low.
In 13+ months, I have seen "Green Bars" only twice. Dallas TX is fairly flat land and most of my driving is "around town." It is well to remember that the computer-controlled traction battery level of charge is limited both ways, never fully charged or discharged, so the battery functions in a very comfortable charge level. You might say "The traction battery leads an easy life."
As for the MFD "mileage" - I find mine over 95% accurate when compared to noting the odometer reading vs. the gallons pumped. Usually, the MFD reading is optimistic.