2. From NPOV: "NPOV requires views to be
represented without bias." If
the only source we have is first-party, the article will be inherently
biased, as it is nearly impossible to write fairly and neutrally about
oneself.
Each of your steps is true most of the time, but occasionally not true
(you
even had to admit it in the one quoted above, by adding the word "nearly").
The derivation won't work, for the same reason that Holmes won't work.
"It's damn near impossible to write objectively about yourself or
something you have a vested interest in promoting" has a lot higher
certainty than 90%. I'd put it somewhere around 99.999%, and even that's
generous, that's saying 1 in 100,000 people could do it.
But that's not what the statement is saying. The statement is saying that
if the source isn't objective, the article written from the source must
also be non-objective. This isn't true anywhere near 99.999% of the time.