Thank you for your response.
Getting a multi-state agreement to drop winner take all would be ideal, but is not strictly required, but I take your point about how unlikely it is. I will also note that it would be easier than the Constitutional Amendment that would be required to eliminate the Electoral College.
It would add roughly 100 Congressmen, I will note that the number of Congressmen is set by law, not the Constitution, it was regularly increased until we had the influx of southern and eastern Europeans started arriving in the 1920’s. Suddenly the fear of immigrants taking over the US led to a freezing of the number at 435. If we had the same ratio of Congressman to population that we had in the 1810’s, there would be over 6,000 of them.