The canned answer is not clean it and get over it.
My first two 7100's were replaced due to thier issue!!!!
So yes, it was a REAL ISSUE!
If you have not had this issue, that is great.
I hate to say it, but yes, the answer may just be get over it.
Here's what we don't know...
- How clean (I mean, really, honestly and truly) is the environment you keep your cameras in?
- When you lay your camera down, how is it oriented - lens up, lens forward or lens down?
- When you shoot outside:
- What do you shoot?
- Where do you shoot?
- Is there grass and trees and dirt and pollen?
- Do you face the camera up and shoot at the sky?
- Do you wipe your camera down when you come back inside?
- How long does it take you to swap lenses?
- When you take the old lens off, what do you do next, and how quickly? And what about after that?
- Where do you keep the back cover when it's not on a lens?
- How dust free is the environment that you "clean" your camera in? Do you have forced hot air or air conditioning running when you do this (moving dust particles around)? Is your camera lying face up on the table after you're done with no body cap for anything more than a second? When you blow out the chamber are you holding the camera face down to allow the dust to actually escape using gravity to pull it away?
It seems like you are doing your best to take care not to introduce dust into the equation, and that's great. But, a) the D7100 has been around for a while with no reported symptomatic issues such as what you're seeing, and b) you're on your 3rd camera, so at this point given the 2 factors mentioned I would say that it's got as much or more to do with camera use (and be clear, I'm not saying improper use) than the camera itself.
Could there be a dusty box of cameras that went to a Best Buy near you? It's not out of the realm of possibility. But I wouldn't put my money on it.
I'm not trying to place blame on you. I'm trying to assess, or at least get you to assess the conditions under which these 3 cameras have been used.