Great Idea dennis372614.
The way template works is you update the template page, and then when you have DW will ask you if you want to update the other pages. You should click Yes. To see the update, you should only update the other site pages to your server, you should not upload the template page.
for your second question, if you are planning to update the product results or the product detail page, you will first need to update the product insert page in the admin to add your new fields.
In regard to the waring, it looks like you have powerstore in a subfolder. What you need to do is create a new site in DW, and set the root of the site to be the root of the PowerStore folder so the WA_eCart folder is in the root of your new site.
let me know if you still have any questions.