OS levers that aren't cross top?

understandable why nobody even bothered to produce an "indipendent flat bar lever" with a 31.8mm clamp

Well to be fair, two companies do as I demonstrated above.

they could never be sold or equipped on an off-the-peg bike

A quick google finds a multitude of drop-bar equipped, 'off-the-peg' fixed gear bikes with exactly this set up.

ISO standards require to have brake levers where they can be reached from the furthermost point on the handlebars.

If this is accurate, then the fixed gear brake levers I posted above are no less compliant than standard drop bar levers as neither can be reached from the furthermost point on the handlebars.