Hi Nelson,
For the question 1, after you migrate the users to Office 365 and point the DNS records to Office 365, they are able to access the emails through Office 365 account.
For the question 2, if you have 6 users and the 6 users need to use Office 365 services, you need to pay for 6 users.
For the question 3, if you don't continue to use a trial version, you need to remove your customer domain in Office 365 Portal, and then change the DNS records to the original.
For the question 4, if you have added and verified your domain to Office 365, and created the DNS records for Office 365 services in your domain hosting provider, you won't migrate the users to Office 365, they won't be able to receive emails.
Thanks,
Edward Qu