As explained above, with the reseller system *you do not need to log with each registrant handle*. If you have created the domain name (and registrant handle) while being logged to your reseller account, *you can change anything from your reseller account*.
So I can change from the reseller account, without logging out and back in, the complete contact details of a domain managed by me but owned by one of my clients?
Moreover, in the reseller system, all the invoices are sent to the reseller. Have you tested the reseller system?
Exactly what shouldn't happen! The invoice needs to be in the name of the owner, not in the name of the "reseller".