Googling 2012 3 series reviews this is what I found:
http://www.thecarconnection.com/overview/bmw_3-series_2012
http://www.edmunds.com/bmw/3-series/2012/
http://usnews.rankingsandreviews.com/cars-trucks/BMW_3-Series/2012/
http://www.themotorreport.com.au/53891/2012-bmw-328i-luxury-review
http://www.autocar.co.uk/car-review/bmw/3-series
4 pages of google and I can't find the ratings you showed, source would be great. Until you can give a source to your ratings as far as I can see there is not a single bad review for the 2012 3-series to back up your claim of diminishing quality in BMWs. It seems your perception of BMWs bad quality comes from the defects in newly introduced models (which are always fixed one or two years after release), that said Tesla has had way worse things go wrong with its cars... From the 3 reported spontaneous combustion of the car, to the dozens of reports of the car randomly stopping on the road or not even being able to start up in the first place, to the horrible battery drainage issue that was only just fixed a few months ago... they're really not as perfect as you seem to think they are.
You can't get any info on reliability like that; you have to look at compiled data, rather than product reviews. I actually got the data from one of the sites you've linked, except it was on their 2014 model page: http://www.edmunds.com/bmw/3-series/2014/reliability.html
It is impossible to know the reliability of a car soon after its release by just looking at said model year. Instead, predicted reliability is analyzed based on the defects trends found in previous model years. Hence, the outlook for 2014's model looks pretty bad, judging by the history. Of course, there's always the chance that this judgment is unfair because they've built the 2014 one much better, but this is unlikely.
If you wanna see more in-depth analysis of this sort, you should check out the Lemon-Aid books.
There's a lot of complaints about defects from owners, too. Apparently failing water pumps are relatively common.. that kind of stuff is just silly.
The Tesla can't be perfect; it's a newly designed car. That said, it seems to have had few defects from what I could tell. The "spontaneous combustions" you've looked up were stories that were widely overblown. Haven't looked into it in detail, but I recall a few cases where it turned out that a minor fire has started as a result of an owner driving over a large object, damaging the battery pack. The "vampire drain" issue was more of an annoyance, though it did look bad on Tesla for taking so long to fix it. Either way, it's pretty minor compared to what goes on with the other car makers. We'd need a couple years to compile proper stats, though.
I'm actually curious now to compare the two cars center of gravity, considering the thing that makes ///M cars so great is that when designing the cars they put so much emphasis in flawless handling that low center of gravity is a huge priority. Also although it's true that the Tesla can optimally distribute mass throughout the car, it weighs 4647 pounds while producing 416bhp and 443 pound feet of torque. The m6 weighs 4244 pounds while producing 560bhp and 500 pound feet of torque but is in the FR layour (which isn't necessarily bad, but it's not as good as complete control over weight distribution) . The point being, although Tesla has greater control of mass and an arguably better traction control system (not denying or agreeing with this one since I don't know enough about it), it still does not out perform BMWs. I'm not saying this will never change, but as of now BMW still beats Tesla in terms of performance, as you said, the m6 is built for the track.
I'm sure the M6 has a low centre of gravity and great handling compared to other gasoline cars, but you can't get around the fact that the engine tall and as such has to be mounted higher. In the Tesla, the motors are tiny and basically mounted flush to the floor of the car along with the battery pack. Dunno if anyone published centre of gravity specs for both cars, but I'm pretty sure you'd see a big difference. Some car makers have gone to great lengths to try to get an advantage here, like Subaru with their flat Boxer engine, for example.
Dunno how a M6 compares to a M5, but I came across this the other day:
[video=youtube;vvHTN0Yi1t4]http://www.youtube.com/watch?v=vvHTN0Yi1t4[/video]
The main advantage of the electric drive is its massive power output right from the start. Literally makes the M5 look like it was just sitting there. Of course, the M5 eventually (almost) catches up once they both pick up speed, due to it having more HP. Had the drag race been longer, the BMW would have won no doubt. Had they thrown some sharp corners in there, though, things would look different.