I'm pretty sure we can assume the Andalites don't lactate, at least in the way mammals do.
Also, just because the wishflower implies a longer period of time does not necessarily imply vivipary; it might just mean that an egg may take a long time to hatch, and they are hoping it does so successfully.
Or, they could have such a strange method of reproduction that it is difficult for us to understand it without it being described to us, simply because it is so different from more local means.