Questions for Migrating to Web Payments SDK from SqPaymentForm in ASP.NET

We would like to make the change now so we don’t have to worry about it later when the SqPaymentForm is discontinued.
So, Plan B would be to use the javascript version of the Payments API.
Do you have any solutions to the issues I was finding for questions 1-3? I’m sure the line with the fetch command is wrong, but I’m not sure of the correct way to use it for Square.
Thank you.