1 - About 1200 is where they used to place you with no ranked exp. That is prolly where they still do. High Bronze / Low Silver players.
2 - Duo Queuing does two things. One, it allows you to affect the matchmaking system based on your partners MMR. So, if you pick a Duo Partner with the same MMR as you, you will face players around your MMR. If you pick a partner with a lower MMR, you will play with players of lower MMR (on both your team and the enemy team). If your partner has a higher MMR, you will have players of higher MMR than you on both teams. It basically (Your MMR + Partner MMR)/2. This is matchmaking.
bonus Duo Queuing also affects LP gain. If you are playing against a team with a lower MMR than you, you will gain less LP(LP losses will be higher) b/c you are against weaker opponents. If you are playing against a team with a higher average MMR than you then the LP gains from winning will be higher (LP losses lower).
3 - That is up to you. You know your friends better than me and now you know how their MMR will affect yours.
4 - There are reasons to not duo queue :
- Your MMR is changed (even if slightly).
- You may not learn as much, many duo queue's play together enough that they subconsciously cover for the other players weak area's of play. So you do not need to learn how to play those area's of the game better yourself as you always have somebody doing that for you.
- You are adding a second consistent factor to your games. This can be good if your partner plays well and does not go on tilt etc... However this can be harmful if your partner is having a bad day (play-wise or otherwise) and you may have to awkwardly not duo with them because they just are not doing well that day.
- I think of solo queue as more of solo improvement, so I like to limit the constant factor to just myself and my play. (Although I will duo, just usually not on my main).
5 - This was a coincidence. Ranked used to be ordered from highest to lowest MMR as Kazaashi stated. However, this was changed due to players treating anyone in queue below them as a *baddie / scrub * because they had a slightly lower MMR. Team captain is now picked randomly. Each player gets a 20% chance to be team captain, if a duo Queue is involved, the Duo has 40% chance and the player who started the party will be captain.
6 - Counterpicks are not really a thing. Not hard counterpicks anyway. There are too many other factors. In a vacuum, yes one champion may counter another. This really just means they have a better kit against the other champion. However, the kit does not take playstyle, skill level (with each champion), jungle pressure, roaming, summoner spells, items, etc into account. If you are very good with a champion and you know how to play the match-up, you can beat your counter (as most counters are soft counters).
An example, 
is considered a counter to 
. can destroy in lane, rendering her useless. However, if plays the match-up well and makes it to level 6 not too far behind, she can all in and destroy the . Does have a built in advantage at the beginning of the game? Yeah. But can still play the match-up well and come out dominant if she has more skill than and knows the match-up well enough to play it correctly.
General Tips -
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To follow on the last point, game knowledge and familiarity with champions will take you far.
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Duo Queue is a double edged sword, it comes with benefits and drawbacks. It just depends if you have more fun playing with a friend. I wouldn't Duo with somebody unless you genuinely enjoy playing the game with them. There will be good and bad games played by each of you and at some point you will end up costing the other person the game (and vice-verse).
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If you can, watch a replay of your games or pivotal spots in your games after you have stepped back from that match. So maybe a week later? This will allow you to more clearly see mistakes that you made and decisions that could have been made to improve your game (whether you won or lost).
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A positive attitude will go a long way. You don't have to be a cheerleader, but don't look for the worst in people. Even if someone is a jerk, just mute them or pretend they are having a horrible day and they are just venting. Report them after the game if you see fit.
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Don't be afraid of "I'll report you!" threats. Nothing bad will happen unless you get a lot of reports consistently.
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Have fun in ranked! It isn't nearly as scary as most people make it out to be.
