IMHO, we genuflect before the tabernacle because Christ is actually present. We reserve this action for Him. Blessed Mother is not actually present in the statue so it may not be an appropriate gesture. The statue serves as a reminder of her, much as a photo of a family member might. We don’t genuflect in front of paintings of Christ, do we? This is the same kind of thing.
As to posture during prayer, I would say the best posture is the one that gets you to pray. If standing works, great. If laying down works, super. The key is to pray, pray, and then pray some more. Obviously we have postures for prayer that we use in mass and other public venues, but at home, go with what’s going to get you to pray.