Checkout Page Customization

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We would like to develop a checkout app that helps users from certain regions to check out in localized mobile payment gateways. 

1. Can we customize the check out page for merchants and only show it to users from certain regions of the world? If so, which API/Theme would we need to look at to achieve this?

2. Can we use links or other methods to redirect the users to check out using third party systems? 

3. Can we display popups to users from certain regions on homepage/landing page to inform them of how we support their region/country? If so, which theme/API should we look at for this?

Thanks in advanced!