Write to different schema with own tablespace from another account
Quota calculation is based on table owner, not on the user that inserts (or deletes or updates) data.

For example if u1 creates a table, u2 inserts 300MB of data, then u3 deletes 100MB of that same data and u4 updates 100MB of that data, only the quota of u1 is checked, for other users, just permissions at the table level (grant insert/delete etc) are checked.

In the example you give, there is no contradiction, u2 inserts into the table owned by u1, so only the quota of u1 is checked.

Note there is no need to specify QUOTA 0M as the default is not to allow quota on tablespaces.

http://docs.oracle.com/cd/B10500_01/server.920/a96521/users.htm#15298

You can assign each user a tablespace quota for any tablespace (except a temporary tablespace). Assigning a quota does two things:

Users with privileges to create certain types of objects can create those objects in the specified tablespace.

Oracle limits the amount of space that can be allocated for storage of a user's objects within the specified tablespace to the amount of the quota.

By default, a user has no quota on any tablespace in the database. If the user has the privilege to create a schema object, you must assign a quota to allow the user to create objects. Minimally, assign users a quota for the default tablespace, and additional quotas for other tablespaces in which they can create objects.