Note: I started to jot down a response here and then saw your comment about another thread and provided answer. I went through the question, read the answer, only to realize it was actually me who responded to your original question. The thread is only about two months old, and I clearly forgot about it - so no worries if you feel like you repeated yourself - I didn't catch it either! I am also reusing the same examples!
First, AKA: For me, to 'deviate' from a name is to take on an alternate form (slight spelling difference) from the original. As an example, my ancestor Josef Volcik was born in Vsetin, Moravia where he married and had two children, also Volcik's. After the family migrated to the United States, Josef and his wife had two more children - but their surnames were spelled Wolcik. I won't go into the details of why the name changed, but I would consider this a derivative of the original surname - making them one in the same (to a certain extent - at least in my line).
A variant on the other hand (again, this is just my opinion), would be where the surname is one in which many people from a different culture would have trouble pronouncing, spelling, etc.; so a different (either acceptable or entirely made-up) form is used. For a native English speaker, trying to pronounce a name like Anwulichukwu can be quite a challenge, so the name may be shortened to "Ann" - which brings up a comic strip I ran across a while back:
This would also be the same for a person who goes by their middle name in place of their first name, etc. So, I would be of the opinion that AKA is suitable for variants, but not for derivatives (at least as I have described them here).
Second, LNAB: I agree wholeheartedly with your statement and it is very similar to what I posted on the original thread - so nothing further from me here.
It is possible, of course, to enter a "Current last name", suggesting that it was one way at birth and later changed. I think that would be useful when an actual spelling shift occurs from one generation to the next, but not so much when it was simply a correction of judgment.
I would venture to say that the current last name field should also be used for official/proven information. For instance, if the LNAB at birth (according to birth cert) was Wolcik, and later in life the name was officially changed to Volcik (to correct the spelling, etc.), then the Current last name would be changed to Volcik so there is no confusion.