well, the checkout wizard uses a newer style of checkout expireince where the express checkout button is part of the main checkout form and uses ajax.
the eCart -> Insert Checkout form access point uses the older style of checkout experience where the express checkout button is in its own form.
if you want the new experience, use the wizard to create the pages.
if you are going to create the pages manually, it will be created using the older checkout experience, you will need to do some manual configuring to move the express checkout button into the checkout form to use the billing fields for passing the info to paypal.
As for the form element naming, I Would recommend leaving the existing form element names rather then changing them. thats not to say you can't change the labels to change the wording that is showed on the checkout page, but the element names should be left as is.
Actually, in your screen shot, it looks like only the label is being changed, the label has "Street" but the name of the element is still "Address_1". the label only effects what is displayed on the form on the checkout page, the name is used to refer to the element in the code on the confirm page.
With the API Credentials, it depends entirely on the account. if the account is set up for PayPal Payments Pro, it should work, but if the account where set up for Express Checkout only, it won't work.