Essentially you are correct. 501-506 can be used for domestic arrivals. And yes, it is very congested. Of course, in theory any stand can be used but the passengers will need to be bussed to the T5A domestic arrivals area.
Now that BMI is in house there will never be enough room at T5 to accomodate all the routes - we'll be operating out of T1 and T3 indefinitely.