You've used the word 'advantage' before and, as a mathematics graduate, I don't like that term simply because it has no universally defined mathematical meaning.
So, with reference to your first paragraph, I firstly agree that a player with an ELO of 2000 should be expected to win 64% of games against a player with an ELO of 1900.
However, if was to try to assign a meaning to 'advantage', I could either get to a figure of 28% (not 14%), because 64% exceeds 36% by 28%, or to 77.77% because 64 exceeds 36 by 77.7% of 36. (The second definition would make far more sense to me but, as I've stated, mathematics simply doesn't recognise the word 'advantage' in the way that you've used it).
With reference to your second paragraph, the difference ISN'T linear. As stated in the third paragraph of my Wikipedia link, where their ELOs vary by 200, the stronger of two opponents would be expected to win 76% (not 78%) of games against the weaker opponent. Depending upon which of my two definitions (above) of 'advantage' that you choose to use, you either get to a figure of 52% or 216.67%.