Solution for the URL Alias and question marks issue with Webform
By default, webform will redirect you to an internal URL with the submission parameter sid. The only way to prevent this parameter to being displayed is to redirect the user to an external URL.

What you can do is set up a bit.ly short URL that targets your thankyou page, therefore webform will think it's an external URL, won't add the submission parameter and the user will land on your http://www.somesite.com/thankyou page.