I'm a newby too, but have several weeks experience, so I can answer some of your questions.......
#3 - Batteries with an LED are not necessarily automatic. The LED just indicates it is delivering power to the atomizer, and manuals can have one too. IMHO the automatics are too sensitive and moody. I have a manual on order, and all the old-timers around here seem to swear by them.
#4 - I haven't tried a cart/atty combo, but they are designed to be disposable, so I doubt the filler is good enough to reuse much. Other than that, you can pretty much refill any cartridge. I bought a whole bunch of pre-filled carts for my pen-style (801), which turned out to be a mistake, but also a learning experience. It was a mistake, because the carts didn't deliver very well, and I kept thinking I was doing something wrong. Then one day I took an old cart, washed everything out, let the filler dry, put it back togther, and refilled it with juice. WOW!!! What a difference! I can actually enjoy my e-cig now instead of messing around with it all the time, and I get good consistent draws with it. I am only buying juice and blank carts from now on. Lesson learned.
As for #1, I have only tried a pen-style so far, but I have pretty good results with it. I got mine from vaporcig, but after more research I'm pretty sure I could have gotten the exact same thing cheaper. I wasn't getting consistent vapor at first, but after making my own carts I'm pleased with it other than the moody automatic battery. I am currently waiting for my Janty Stick (manual), and have high hopes for it.
And for #2, I think the 501 styles are closest in size to analog cigs, but there might be others that I'm not aware of yet. However, the smaller the e-cig is, the smaller the battery is, and I can definitely tell you that you want a nice powerful battery. I would be leery that when you are first starting, a smaller one won't give you enough satisfaction to stick with
vaping.
HTH!