Can app functionality control 'Add to Cart' buttons and checkout extensions?

[triplesingle]

1. Is it possible develop a functionality within a Shopify app so that the "Add to Cart" buttons for certain products are greyed out until a certain condition is fulfilled by the buyer?

2. Is it possible when developing a Checkout Extension for the Checkout Extension to retrieve query params on the checkout page? I am not sure there is an API for this....