I do it for all books I've purchased because Fair Use has ruled that I have a right to make copies of media I've purchased, for my own personal use.
Calibre & DeDRM. If these are the tutorials you've tried and have failed with, I can't help you. It's even harder with Linux, but somehow I've managed. You might have more luck if you say how it's failing.
Since when? There was a ruling in the 80's that it was. I admit laws may have changed and I didn't notice, but I also believe corporations are perfectly capable of lying - or using half-truths - to convince people they don't have rights that they really do.
DRM & DCMA. Well, I don't know whether this has yet been tested in court, and with the current Supreme Court it might not be wise to risk it, but eventually with a more moderate court I would expect a similar ruling as the original ruling was meant for. A corporation suing an individual for circumventing DRM with no proof of redistribution, I think, would result in a ruling DCMA would not like to have on the books. But, that's just me, and we don't have a moderate higher court, and you're right.
I used to think the same thing, but I did an effort post about this about a year ago (here's the link)
The article you linked to says something similar to my own understanding: basically, DRM circumvention for personal use is officially not allowed under DMCA and could absolutely be used against you in court, though the likelihood is low. The exceptions the author mentions are pretty nebulous, and the Library of Congress actually addresses the most common cases in their discussions and publication and affirms that they are not allowed.
I don't personally agree with their interpretation, but I think more people ought to know that it's officially not legal to circumvent DRM for personal use.