[Reply:}The fact that Jesus said that John was less than the least in heaven is irrelevant here. Both John and Mary were on earth.
But Jesus was also on earth at the time; and He said "kingdom of heaven", not just "heaven". This is important, for He also said, "the kingdom of God is within you" (Luke 17:21). I take what he is saying to refer to the giving of the Holy Spirit to those in the kingdom of heaven, following the distinction between John's baptism with water and the baptism with the Holy Spirit that he and the NT writers draw in a number of passages (Matthew 3:11, John 1:24-27 and 3:3-8, Acts 19:3-6, etc.). Mary was "overshadowed" by the Holy Spirit (Luke 1:35), and the sound of her voice fills John's mother with the Holy Spirit and makes John leap for joy (Luke 1:41). Regarding Luke 11:28, that can be translated as "Blessed *indeed* are those who hear the word of God and obey it," which would be emphasizing that Mary is blessed because she heard the word of God and obeyed it ("Be it done unto me according to your word": Luke 1:38), not merely because she is physically related to Jesus; that is how I take that passage.
I'm wondering if you're going to address Luke 11:27. Sure seems that Jesus was dismayed by the early displays of mariology.