Yes you can easily change the template from the Google Sheet after you already started to work on any template.
Yes you can! You can use ImportRange formula or just copy the data from your sheet.
You can use embed feature and embed Sheet2Site page to your existing website for example on example.com/page or you can connect it to subdomain like page.example.com.
There is no a free trial but there is a 30 days refund policy. If you will not like Sheet2Site for any reason you can get your money back.