rabbiofberlin,
I think we need to distinguish two things. The literal “ama” of a person, i.e. from the tip of their middle finger to the bottom of their elbow, and the standardized measurement used for building.
When it comes to mikva and mechitza where the shiur needs to serve a practical purpose, the ama will be the literal one and will changed based on the average ama in each generation. When it comes to things like the mishkan or the teyva we are dealing with a standardized measurement. The Chazon Ish can be right as to the standardized measurement used, even though that shiur will not be what is used for mikvah and mechitza.
Tosafos is stating a fact about normal human body proportions. The average human is the height of 4X his ama (elbow to fingertip).