Your list looks good - just a couple of comments on different parts of the list, in case you aren't aware:
Letter from the sponsor confirming the application and explaining the grounds of the relationship
-Intent to marry/Details of engagement
This is not normally part of the sponsor letter, although obviously information about when you intend to marry can be mentioned in the letter. Your evidence of Intent to Marry will normally take the form of correspondence with the church or registry office where you will marry, enquiring about possible dates for the ceremony, maybe even booking a provisional date at the church/registry office. You can also include any other evidence of intent to marry, such as pictures of the rings or dress, details of venue bookings for the reception etc.
Evidence of on-going contact
-Screenshots of facebook, emails, or phone bills.
It is recommended that you do not send anything from Facebook, as it is not considered a reliable source of evidence, and in some cases, can even hurt an application rather than help it (i.e. Facebook accounts can easily be faked). It might be best just to stick with any actual cards or letters you may have sent each other in the mail, plus screenshots of your email inbox/Skype call logs/phone bills etc. - only send a couple of sheets of paper for the screenshots though, you don't need to send reams of paper or anything
If available, a provisional registry office booking
This will be part of your Intent to Marry evidence, as I mentioned above.