Agree completely
First, Insurance, DDs and Splits add to advantage at varying points.
Second, no counting strategy attains 100% efficiency.
I don't disagree. It is just that in terms of % edge at least, this isn't of much import.
Now, if you are trying to grow your bankroll from a small base, such apparently minor details take on some importance as you get a kind of compound interest effect if you know how to exploit such aberrations from linearity. Sadly, this is probably something very few players consider or exploit probably. Moreover, this is true of the many fine details of blackjack, and you have to draw a line under the practicalities of applying your blackjack strategy at some point.
Regarding 100% efficiency-you are right, the complexities of non-linear advantage have an important theoretical property. It is essential to understand this stuff if you are going to do research into the mechanics of blackjack. I think Francis is focusing on the implications for regular counters though.
Btw I wrote an entire book essentially based on the principle of non-linearity for baccarat card counters, so I'm certainly not down on the whole concept. It is fascinating what happens with blackjack (and baccarat as Griffin notes, and indeed all similar card games) when the deck structure breaks down at close to 100% penetration, and the EOR's go out the window.
An additional point: is advantage &risk/ TC linear? I would think not.