You're much better off playing 6 min quarters. It's not like you'll end up with double the vc for playing 12 mins, you might get for example 1200 instead of a 1000vc. Not really worth it imo. On top of that, now you have a half hour game instead of an hour and you'll get faster to the off days for upgrades and extra endorsement vc. As far as i can tell teammate grade is the biggest factor in how much vc you get and 6 mins should be plenty to get the A+. Of course there's one downside to shorter quarters, it'll be pretty much impossible to get certain HOF badges.
As far as myteam, it's awful. I barely played it this year. Same old problem (super duper over the top rosters with overrated players that have no basis in reality) and it gets worse every year. For me this could have an easy fix, something i've put in the wishlist threads for however long i've been here: a realistically restricted mode and/or roster based matchmaking that actually works. To be clear, i don't have a problem at all with unrealistic cards. However, they still need to have weaknesses. For example: in 2k15 you had a jared cunninham card that was complete fantasy, 6'9 and one of the best dunkers in the game (he's 6'4 irl) BUT he couldn't shoot and had poor defensive ratings. I don't mind that, i actually enjoy(ed) it quite a lot. What i hate is the 2k16 and 17 cards that can do EVERYTHING. Now, it's just a bunch of superstars who have maybe one weakness irl and 2k just goes ahead and eliminates this weakness. I hate it.
I get it though. Myteam is the biggest cashcow and a lot of people seem to prefer putting 10 95+ players together instead of carefully balancing their teams strengths and weaknesses using stars as well as roleplayers. Of course these people consider 90+ cards role players though, so... To each their own i guess. I personally can't take it anymore and the badges make it even worse.