Hi,
For the address_street2 field, you should leave it unpublished and create a new field instead.
For the upload of files during the checkout, you should create a custom field of the table "order" and of the type "file". That will add it to the order automatically and you will have it everywhere.
For the issues you have with the "add to cart" I don't know what it could be. I tried on several browsers and it's not working for any of them on your website. Normally, such issues come from caching or queries errors, but you don't have any caching activated and no problem with the database. It must be something else, but I'm out of ideas.
You say that it now works on your PC. What did you do on your PC which would be different ? Are you logged in ?
If you clear your browser's cache/cookies does it still work ?