There will be some unbalanced games at the start anyway, because we initialize from global. There is no way to gauge how well the very different global experiences translate to skill in a new matchmaker, that's why we increase the sigma to keep this phase short, but it will still be there. In your arguing it seems like you are forgetting that the 4v4 full share queue will basically consist of maps that play roughly like setons for the most part. You already conceded that there is not really an inherent difference between setons and other 20x20 maps with spread out spawns. You still argue about setons specifically, because you think seton's knowledge will translate so bad to other maps to easily make a difference of over 500 rating? Seems pretty inconsitent to me.
What the map pool team could do to play it super safe is to leave setons out of the first pool and only include it in the second pool. This way nobody can blame unbalanced games on the setons bo whores. But imo it is not really needed to do that.