Dammit, FFF, you.....you......you bloody apologist, you! So take that!
Oh, the approx. 3 knots / degree I saw used in various USA based reference texts & online but can't remember where. It's only approximate so your 2 kts /degree could well be more accurate in the type you fly. It could also be related to situation ie static vs dynamic heading control.
Vmc vs Vmca et. al.: The FAA in its 'Airplane Flying Handbook' makes it clear that Vmc can vary and also that the published Vmc is under a particular set of conditions. Generally I think context provides enough distinction as to whether one is referring to the published or 'at the moment' Vmc. Published Vmc is only a guide because unless every certification criteria is met then it is unlikely to be correct for the moment. I tend to use Vmca/Vmcg to distinguish between themselves if there is likely to be confusion about which one I mean.