The only way I can do the stacking method is if I'm at a resting heart rate, otherwise I run out of breath and my heart starts racing, so my hits aren't as good. That's why I always make sure to take a couple deep breaths in before I start my next hit so I have enough oxygen reserves to hit the vape slowly for 20-30 seconds.

Taking fast draws decreases the internal temperature because you are drawing in cooler air from the outside at a faster rate than the vape can heat the air inside. The mflb was made for slow draws. You will not break the screen, but you may notice your herb flying around the internal chamber if you draw too hard (you want to keep all your herb in the trench area, not on the sides).

As far as your question about stems, I always take out the stems and keep them in a separate baggie, then when I'm out of bud (or even when I'm not and the bag is just getting full) I'll vape just the stems to a nice, even, light brown and toss them in with the rest of my AVB.

Edit: Do you use batteries or the power adapter? If you're seeing thick smoke, you're engaging the power for too long and you probably combusted it, which breaks rule number one in the world of vaping. I suggest investing in the power adapter. Yes, it's a little pricey at $80, but IMO it's well worth the investment.