Grigore,
I agree with you about most points about CA, The help doesnt helps when it comes to help to develop an app using CA.

About your questions:
a) To add and CA in the dataenvironment in Form designer, keeping the settings you have to,
Right-click in the DE of the form and select Builder, select 2° Tab and click the Add button
and select the CA from the class.

b) I use a CA with Join, but never tested using a subclass of an CA class, In my case I allways call
Stored Procedures at server side using the CA, so I just pass those fields as parameters.

c) I have a diferent aproach to use Transactions with CA as you have and it works fine for me.
At Server side I have the Stored Procedures for Insert/Update/Delete that CA calls with Tableupdate.
Heres a simple model.

= SQLSETPROP(nID, 'TRANSACTIONS',2) && MANUAL

IF TABLEUPDATE(1,.T.,table_name)
   = SQLCOMMIT(nID)
ELSE
   = SQLROLLBACK(nID)
   oConn.GET_ERROR() 		    && Get error from Server..
ENDIF

But CA really could be easier for developers and also needs improvement.

Peter