User visibility of other users

I am using Zoho CRM with Territory Managment in an attempt to make the CRM become a PRM (Partner Relationship Managment).

So far, it works exceptionally well, and has some excellent features (Call logging on the Android ! Superb)

However I am missing two final aspects:

1. By default, stop a user in one Territory being able to see a user in another Territory (visible under when changing Owner on any section). Possible ?

2. As we are a global provider, we occasionally need to share an Opportunity with other Partners in other regions.
How can this best be achieved on an Opportunity by Opportunity basis without sharing all other aspects ?

I know Q2 appears to contradict Q1, but the sharing of an Opportunity would be by ADMIN, and in Q1 I only want to restrict visibility..