I know nothing of Flex takeoffs. I understand the effect of thrust/speed on yaw & roll control. It is an effect at slow speed. VMCG is an on ground scenario. VMCA is airborne and again slow speed. If airborne and above V2 I suggest you should not be anywhere near VMCA. If I have the yaw & roll under control, but the ROC is suffering for an unknown reason, I feel capable of slowly increasing thrust to try and solve that problem while retaining control about the other 2 axis. That is what I've been trained to do, and in light twins, non perf A, was a regular airborne training manoeuvre with new pilots on type. Educating and illuminating, too.
I do not advocate increasing thrust as a common technique during an EFATO. (I have flown for one airline whose CP did advocate that as an SOP). What I do advocate is not teaching that it is not allowed.