1. I'm unsure what you mean by 'early' and with regards to a USB stick. Whatever the source is they need scanning to the website whether that's by USB, desktop, laptop etc.
2. You can scan multiple pages. We scanned our required documents as one file (i.e. multiple pages). For instance, a 4 page bank statement would be one file.
3. There is no strict labeling convention but just so they make sense (eg. Applicant_Passport, Sponsor_Declaration, Applicant_Correspondence_Mar_19)
4. You need every page of applicant's passport and I'm unsure on sponsor. We did every page of sponsor's passport just for completeness