Putting aside minor variations, what is the approach adopted for numerical analysis in the case of OEI bank angles greater than 15 deg ? NFP at commerical weights is going to be negative and to address this at light weights is going to make critical takeoffs somewhat non-viable ?

Yes, probably sufficiently non viable that most operators will opt for 15 degrees only, unless the terrain is really tricky (PNG, Peru?). From NZ Part 121:




BANK ANGLE SPEED GRADIENT


15° to 19° V2 1 x Aeroplane flight manual 15° gradient loss

20° to 24° V2 + 5 knots 2 x Aeroplane flight manual 15° gradient loss

25° V2 + 19 knots 3 x Aeroplane flight manual 15° gradient loss

Interesting .. but I think I would be seeking to organise a more usefully accurate penalty via concession .. that's a bit broad brush for me.