Concerning "whosoever believeth in him" and the Greek. This is an area that I know very little about.
I must admit that "Whosoever believeth in Him" compared for example.
Young's Literal Translation (YLT)
16for God did so love the world, that His Son -- the only begotten -- He gave, that every one who is believing in him may not perish, but may have life age-during.
I really do not see a lot of difference either way.
Well, I do see a huge difference.

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whosoever connotes an indefiniteness, potentiality, possibility, open-endedness, unlimitedness
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believing ones connotes definiteness, selectivity, limitedness, preference, particularity
My understanding of the text is that God sent His only begotten Son to rescue a particular group of people, the believers in the world, from the judgment which was to come and to grant them eternal life. The Son wasn't sent so that whoever decides to believe will be saved. He didn't come to make salvation possible if someone believes... He came to save His people from their sins and to grant them faith in Him.
If I remember correctly the A.V. was translated from a different translation of the Greek (Textus Receptus?). I would be interested in seeing the literal English translation is from that text and how it compares to the A.V. rendering.
The TR and WH texts are identical.
Although I would agree that knowing a little Greek isn't enough and I would also agree with one must add to that knowledge. That is where my agreement with the author ends.
Where in the context of the passage and the greater whole of Scripture does the author see that a "rhetorical strategy" in the verse?
- I am NOT by any standard a "Greek scholar". However, I minored in NT Greek and took additional Greek studies under Dr. Vern Poythress at WTS (Philly) and received a "B+" for my efforts, which is no small feat at WTS during that time; late '70s - early '80s. So, I really don't feel I fit into the category BG created in order to divert attention from his misuse of the original language.
- I agree that there is more to learning biblical truth than memorizing declensions, parsing verbs, etc. However, the additional knowledge that the author specifically mentions is unwarranted, i.e., "
The interpreter must look for the rhetorical strategy behind the text." Are we to understand that before we can understand the true meaning of a text, we have to look for the 'rhetorical strategy' and only then IF we actually find it (aka: eisogesis), then and only then will we come to learn what the text is REALLY saying?

So, what I see is that if your argument is unsound, your interpretation fallible, and your conclusion not widely held, you use your 'status', educational achievement(s), large vocabulary and a bit of sophistry to razzle-dazzle your hearers so that they are put off from being able to offer any possible criticism.

It is always amazing to see how many different ways people will twist and turn a text of Scripture to fit a preconceived idea and/or to prove some new 'insight' that has been hidden for over 2000 years. John 3:16 is no exception in this case.