1) It depends, there are some little rules that I still find myself asking about from time to time. I'm not the most experienced player in the world, but I feel like I had a better handle on 40k. It was much quicker for me to get up and playing Warmachine after a game or two though, with only minor corrections or mistakes. These included accidentally activating a jack that I forgot I wanted to cast a spell on first, moving things in front of other things and wishing I hadn't once I learned it would block certain things, etc. etc.
2) It depends on how quickly you and your opponent play. I've had games go the same time as 3 or 4 games my friend was playing across the room due to my learning, stalemates, looking up rules or clarifications, and a ton of other factors. If you want to lower the time, lowering the points does the trick pretty well most of the time. I've played battlebox games in fifteen or twenty minutes, including setup. It's usually easier to find people practicing for tournaments (in my area it was usually 50 point lists, but a lot of people play 35 as well) with bigger lists set up on a Warmachine night, but the community's fantastic about new players generally and will take time out to teach you the game or play a quick match.
3) They are compatible and it's very common. I can't speak to tournaments, but I found myself playing against Hordes for months on end at the FLGS (which was bad for my list at the time, so I remember every game very vividly). Popularity has varied in the places I've played and seems to have a local component. If you want to find opponents, call your LGS, they often have a WarmaHordes night you could go and check out. I was able to play the same night I bought a battlebox because a guy who runs Ret wasn't using the figures and let me use his.
4) There are a lot of lists that can utilize lower model counts within factions, but the ones I'm most familiar with having a high model count would be Retribution and Khador. Especially Retribution as you're going to be running UA in addition to every unit you run (usually at maximum troop size for the unit) which just further increases expenses and model count. Khador runs a lot of infantry as well.
5) It depends on where you buy them. I used bartertown.com and was able to pick up more than I would ever need in my faction (for 50 point lists with multiple warcasters and variants) for around $250. You could probably get a decent list together for under $75 if you tried really hard and had some luck on your side. The thing about Warmachine that makes it cheaper than other wargames is the fact that they don't drop a new book/necessary model/whatever on you every month or so. Once you buy the models, you can keep using them until you're interested in trying out new ones (not counting the weaker stuff that doesn't see much table play. Not that they're necessarily bad, there's just usually something in every faction that has better alternatives that fill the same role for the same amount of points or less). Nothing has incredibly changed in the past year from what I've seen. Compare that to 40k where I felt like after a year you'd be better off throwing your models away and starting from scratch than trying to add to them. For me, it's less about the cost and more about the reduction of future costs.
7) Nicer than any other tabletop gaming community I've run into. They've been extremely patient, helpful, and just downright pleasant everywhere I encounter them. Page 5 is to keep people from acting like tools, believe it or not. A good way to passive-aggressively bother another person playing tabletop games is to run, hide, and slow the game down to gain an advantage. That's not fun when you can't just open up candy crush and ignore your opponent. In addition, it's a cheap way to run down the clock in tournaments if there's no rule against it. The wording is poking fun in a good spirited way, trust me. It's basically saying "have fun, it's a game" in a way that pokes fun at the people who play like tools.
8) The rulebook that comes in the 2-player battlebox is actually better than the regular rulebook. Because they put in only the parts you need to play and cut the fluff. It's good to have both, but if I could pick one (considering it's a reference book for playing the game), I'd take the one that wasn't the size of a textbook. Past the 2-player battlebox, the battlegroups don't come with rulebooks.
9) The faction books (the books sold separately that have the name of a faction on them) have fluff and other helpful information for each faction. They're not required to play. The models come with stat cards for the units you're buying. They list out the hit points, statistics (strength, speed, health) and weapons/abilities/magic used by the model.
10) It depends on the army box. If you're talking about getting the all-in-ones, short answer: if you like the way the list will play (once researching it, of course) yes. Most of them come with great jumping-off points for the army and standard units/jacks/warcasters you're going to reuse frequently. As far as the battleboxes, my opinion is: no, depending on what you like to play. It's certainly cheaper than buying two jacks and a caster. It's also a great way to test the rules out, and a lot of people have more than enough to play battlebox games (straight out of the box games with no scenario, just a quick game with one objective: killing the other player). But if you're not going to use what's in the battlebox, I don't see why you should buy it. I spent $50 on my Retribution of Scyrah battlebox and used the models inside twice, tops. Once the night I bought it and once another night between building my other stuff. It's much easier to watch a game or two and ask questions.
11) Read around the sub a bit. The people who post here are crazy helpful and take a lot of time to help out all levels of players. Check out some YouTube videos that feature factions you're interested in, there are a ton of battle reports and the like. I recommend WarGamerGirl for new players of all factions, even though her focus is on Khador. She does a great job of explaining what's being used and why, plays at varied point costs against varied factions, and uses great visual aids (mock-ups of health boxes or ability text) to represent something when making a reference. She also speaks clearly, which is a huge bonus point for me. Too many times when watching a battle report I'll find myself rewinding several times to catch something (lingo, nicknames used out of context, mumbling, in-joking, etc.) and it really throws me off.
Like I said in #2, it's really going to depend on how comfortable you and your opponent are with the rules. There are a lot less little things to remember, and most things are printed on your cards for quick reference, but expect there to be a few "wait, hold on"s before your games start smoothing themselves out.
I hope that helps. I'm sure there's a lot I missed, but maybe someone else can catch something I forgot to mention or say it in a better way.