How to preserve MacbookPro battery from degradation? Platform: OS X: Yosemite 10.10.4; MacbookPro 15" Retina (mid-2015).
Fact: There is aging for battery in any case - using or not. But while using - there is difference of how to use. My questions - in order to minimize battery degradation: 


How often should I totally discharge battery ?
Is degradation rate is a matter of full cycles or it doesn't matter
to what highest and lowest level I charge or discharge laptop ?
Most (99%) of the time I am connected to power - which leads to 100%
charge level always. But many sites say, like following link:
http://www.runpcrun.com/laptop-battery-use-and-storage-guidelines
say, that better to keep battery 40% charged than 100%. How to
achieve that (there is no ability, as far as I know, to manually
limit charging battery level) ?
Do you have empirical evidences to prove or disapprove this
expectations ?
I ask this since to replace battery in Macbook Pro is hard to be
done - many screw types, many modules do disassembly, using glue to
grasp components without moving inside. And therefore I prefer doing
this rarest I can. How much time (how many years) it typically takes
upon your experience to get to buttery degradation of 40% capacity
compared to Day-1 of using your Macbook Pro ?
After 2 minutes of laptop using - the internal temperature (CPU,
RAM, Battery) gets to 85-90 degrees (Celsius) and stays so till I
turn off (sleep) laptop - for 12 working hours every day. Is there
something I can do about heat (both in general, and regarding
battery especially)?


What else should I do or know to not be frustrated later ?
Your battery life is based on age and cycles, not so much what you use them for.  Your battery has no idea if you are streaming music from your favorite service or your are crunching numbers to calculate ∏ to the nth digit or just surfing Stack Exchange.
The more draw you put on it, the more it is going to discharge requiring you to plug it back in to recharge it.  Basically, the more you use, the more charging cycles it is going to go through.  Video uses more CPU, so more draw, so more recharging.  Sleeping, virtually no power draw, so less recharging.
You can get good info on battery life directly from Apple:  www.apple.com/batteries/
So, how do you lengthen the life of your battery?  Reduce the number of cycles it going to need to go through.  Optimizing/Updating your software, turning off services can all help; but only so much.  Paying attention to the ambient temperature and not operating it in extreme conditions definitely helps; but I doubt you are in the Arctic or in the middle of the Sahara so this doesn't apply as much as you would think.  However if that's the case, battery life is not your biggest concern.
In the end it comes down to you are going to have to replace your battery.  When is the question and there is no prognostication for that.  If you have work to do, do it.  If you want to watch a video, watch it.  If worrying about the battery is something that you have to do because you can't (or is very difficult to) afford to replace it when it can no longer hold a charge, this is not the device for you.
Since Apple, Dell, Lenovo all warranty their batteries for a full year and offer extended battery warranty for a relatively small amount, you can probably expect at least 2 years with good performance.  They have already run the numbers and wouldn't warranty something they knew there was a good chance they would have to cover.  Remember, they are betting you won't have to use the warranty service.
My advice if this is really a concern is to find out how much battery replacement is and budget for it.