APEX

Hi, I think that Apex could be a great product even if I'm still a newnewcomer.
But I also found a lot of questions about the use of multiple schemas, and I didn't find any good answer.

To summarize

I have existing schema S1, S2, S3 in my DB
I create a workspace with the name WS1
While creating the WS1, I can ask for a new schema NEW or I can request to use an old one (like S1).

If I create NEW, then i don't see any table from S1, etc ....
I can with the administrator tools (the real one, INTERNAL and not the admin from the workspace), I can assihn one or more additionnal

schemas to my WS1. Let's say that I attach S1.

BUT:
In sqlworkshop, I can browse the 2 schemas NEW and S1 but it seems that I can't create with "query builder", a query that use tables

from NEW and S1. "Query builder" seems to use only one schema at a time, not the both together.

BUT
In "create application" , I must select only one schema (let'say NEW). But I can see the table of S1, if S1 has granted some privileges to NEW.

So
a) Does-it mean that when a application "runs" here, it runs as if the user was the one we gave (here NEW) when creating the application ? In consequence, you have to grant all the necessary select, update etc ... privilege on the other schemas (here S1) to NEW.
b) Is-it possible to create a query with query builder that spans two or more schemas ?
c) more difficult ;-)

Let's say that S2 give some privileges to NEW on different tables. What is the interest to assign S2 to WS1 because you still need to give the privilege and even if you don't assign S2 to WS1, you are able to make query on S2 in the application ?

a little bit long, sorry
regards

Hereafter my configuration but I think it's useless for this question

Application Express 3.1.0.00.32
ON
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production