1 - we agree. However "legends" such as 85mm-for-portrait do not come up for no reason, good or bad.
2 - one point perspective, seen from the photographer's eye. For a given distance, FL changes mean framing changes, but perspective (angle of view, relative apparent size of objects) does not change. So it is safe to recommend a "safe distance" of at least 2-3m for "neutral" portraits, and choose the FL according to the desired framing (avoiding excessive cropping too). Getting closer means indeed that the viewer will notice geometric "deformations" that may be part of the intent - e.g. big nose or long legs - but if not intentional, they'll mostly turn out to be bad.
Maybe the 85 or 135 "legend" has also to do with technology - limitations of viewfinder cameras, or weight and quality of "long" lenses. With the 300 PF on board, I only hope to get more opportunities to use it for portraits.
concerning flare, I generally do not care much, except when shooting organs against the light, one of my usual shooting circumstances. In such case it is critical. Few reviewers test flare thoroughly. As far as I am concerned (i.e. little), I only know about the flare propensity of those lenses I do use for organ shooting purposes - FL range is then from 20mm to 58mm, exceptionally 105. With longer lenses, I hardly get into situations where flare could become an issue, so I care much, much less.