1. You can do that if you want to, but most probably just add a new flavor to the empty bottle, or rinse it out and blow it dry. If you really wanted to have compatible bottles filled, you could order some from here:
3 ml, 6 ml and 10 ml Dropper Bottles, CandleChem Company, Inc.
2. No, you just squeeze the bottle and suck on the end of your drip tip or whatever you're using, it gets nearly every last drop.
3. Theoretically yes, but in reality not really. I don't know what the science is behind it, maybe it's the AW IMR 14500 batteries (for the mini) or the AW IMR 18650 batteries (for the grand) that make the difference. They come off the charger at about 4.21v and then drop and hold at 3.7v. You notice when they drop under 3.7v and then you put them back on the charger. It might be just how the REO operates.
4. Same as 3rd question here, I don't know, but they do, it must be the consistent regulation of the voltage. Every vape is the same on a REO for me. It wasn't on a 3.7v eGo/Riva battery - sometimes it was, sometimes it wasn't. Cartos last longer too on a REO.
People don't rave about REOs because they want everyone to buy one, we are just happy because we found something that feels like smoking and is as easy as smoking, and it's the same every time you use one. Maybe part of it is the security of having such solid device that you know won't fail on you too. Before I got mine, I felt like I was spending all my time taking off my drip tip, topping up my carto, worrying about burning the carto, sometimes having to vape through over filling just trying to get to what felt right and tasted good, etc., then having to wipe off the end of my battery, screw it into the charger and so on.