The theme says it supports up to WP 3.9 – what is the problem you are getting?
Make sure your code validates fully
The biggest issue is typography – it is not great and very inconsistent. You have a lot of different fonts and sizes going on throughout and this needs to be tidied and simplified.
Editing wise yes you can do what you want.
Extended is more about multiple uses and special uses. You can NEVER resell the template anywhere as it is or even modified on here without express permission form the author
Once you buy a file you can download the theme and modify it to your needs and host it somewhere online all within the regular license.
What you need to remember is you CANNOT resell the theme on here or other marketplaces and for EACH INDIVIDUAL time you create a website using the theme/template then you need a unique regular license.http://themeforest.net/licenses
It is very old school in design and could do with modernising and cleaning up a bit
Typography and spacing needs work
video gallery looks a bit bleak and empty
regular license is sufficient – ONE purchase PER DOMAIN
No to all of those queries. You have to own and have created the original file or have permission from the author to be able to sell it.
Tickets will still potentially take up to a week or at least a few days to reply.
Haing bought more than most on here I can only assume what you are seeing is the price difference between different licenses for the item and then pre paid deposit .v. buy now options.
All of these are meant to be there for their individual reasons. You can read more about licenses and buying options in envato’s support here http://support.envato.com/index.php?/Knowledgebase/Article/View/216/54/complete-guide-to-purchasing-items
It Is probably covered in the documentation and most likely is under theme options (if possible without changing the CSS).
The author does not offer active support for the theme but seems to be answering questions in item comments so maybe try asking there http://themeforest.net/item/willow-a-one-page-multipurpose-theme/7718163/comments