Is V2 the same as VENR for your flap zero take-off?
If yes, then it's a moot point.
If no, then you adhere to the SOP speed for the take-off phase. For the second segment, fly at V2+x (x being whatever correction your company dictates), for the third segment accelerate to VENR, and fly at VENR for the fourth segment.
Remember, that with an engine failure we are concerned more with our
angle of climb to get over obstacles. If we fly too much faster than V2 during the second segment, the angle of climb will be degraded and we may exceed the lateral boundaries that a runway analysis is designed around. I've never seen the sense in accelerating to go further down the line when the immediate intent is to get up high in the least distance.
I was always under the impression that 400'AAL was the lowest acceleration altitude that was permitted.
It is. It can be any altitude higher than that based on local topography or company requirements. For example, Aerodata uses 400' AAE where possible, while Aircraft Performance Group (APG) typically uses 1,000' AAE for minimum flap retraction altitude (e.g. end of the second segment). Other companies will use different altitudes based on what they hope to accomplish (normally, increasing the allowable take-off weight).