I'm no physics major and/or engineer but I believe it's more complex than just choosing the bigger number and simple addition/subtraction. I think you'll want the 12v charger if your fans are 12v even though the amps on your charger are higher than the amps on the fan. From what I 'think' I remember from college is that volts and amps are measured in relation to the resistance in ohms. And I believe that the size, number of blades, general construction, etc. all play parts in how much cfm the fan can move.
To be safe I wouldn't mess around with different volts/amps, if you do you may end up risking the designated cfm's that your fan is actually suppose to be putting out. That is unless an expert comes in here and breaks everything down for us heh.
So... completely taking an educated guess at things...
Q1 - Totally depends (I have 2 80mm fans that pull just as hard as my 120mm fans, but have seen smaller ones pull harder and bigger ones pull harder).
Q2 - Don't know
Q3 - Yes
Q4 - 12v
Q5 - Lower running power on the fans, meaning they won't be able to run at their full potential (CFMs). Say your fan says it can move 120cfm and it's 12v and you're running a 6v. Your fan may be only moving 60cfm now (just an example).
Q6 - I believe it still matters, again I don't know for sure.
For my grow I have 4 80mm fans and 1 120mm fan. I have 2 80mm side by side on the top left corner as exhausts and my 1 120mm on the top right as another exhaust. bottom left and bottom right I have 80mm fans. My 80mm fans move 50cfm and my 120mm moves 60cfm and I have a constant temp range of 77-86 with lights on depending on overall temperature inside the house (mind you, here in New Mexico we've been hitting 100+ days; no a/c).
Hope I helped a little bit.