Items 1, 3, 4 and 5 entail identifying opponents' specific weaknesses and generally inferior ability and exploiting them by deviating from play that would be correct in the absence of those differences. I agree with you that "playing the opponent" is an interesting aspect of backgammon strategy, but I'm surprised to see it asserted that "almost nothing" has been written about this topic. See, for example, a goodly portion of Danny Kleinman's writings, particularly his many essays on chouette play with "Colonel Whiteflag" and "Diana Dialacube." Or see Jake Jacobs' "Can a Fish Taste Twice as good?", a book entirely devoted to the topic. See articles by Kit Woolsey, Phil Simborg, Hank Youngerman and no doubt many, many more. Try Googling backgammon "weaker player". A narrowed search for "backgammon "weaker player" site:www.bkgm.com", for instance, brings up 41 references, many of them on point.
In your item 2 -- "Taking a double with, say, 27% chances ( no gammons ) but very easy to play for weaker opponents" -- is there a phrase missing? Did you mean to say "The computer will give you a big blunder if you pass but I doubt if it is a mistake against a weaker player"?
I wonder if you might give your best argument why or when, in the two positions below, a superior Blue should pass against a weaker White:
(A) Money Game, White on roll, cube decision?
| White | 10 | ||
|---|---|---|---|
                  | |||
| Blue | 16 | ||
| Position ID: 3wsAAPgfAAAAAA Match ID: MAEAAAAAAAAE | |||
Above, Blue's cubeless probability of winning is about 27.2%.
(B) Money Game, White on roll, cube decision?
| White | 95 | ||
|---|---|---|---|
            | |||
| Blue | 109 | ||
| Position ID: ancNAAPWvA0GAA Match ID: MAEAAAAAAAAE | |||
Above, Blue's cubeless probability of winning is about 26.6%.