LetzFetz,
Yup ANO 20.7.1b is the one, 50' in turning flight when more than 15 degrees of heading change.
V2 has nothing whatsoever to do with ground effect.
V2 must not be less than 1.2 Vs and 1.1 Vmca.
For field length considerations V2 will often be chosen as low as possible with the aforementioned limits in mind but not always as it's the climb speed for the first and second segments with one(or more) engines inoperative.For minimising field length requirements it can be slower than best climb gradient speed.
V1 and Vr also have an impact on V2 as V1 is effected by weight, runway length and runway condition ie wet/dry and Vr cannot be less than V1 nor greater than V2.Vr is also effected by VMU and sometimes tyre limit speeds. Vr is a speed which allows attainment of V2 at 35' on one engine or slightly exceed V2 on two engines if rotation is carried out at the proper rate.
MM.