I've been with Writers House since 1982 and have been pleased with them. I started with Al Zuckerman, whom I met at a writers conference. I stayed with him for several years, before moving over to another agent in the house who more represented my goals and expectations at that time. My association with Writers House has always been a positive one.
If Al Zuckerman is enthusiastic about your work, he will work with you to hone it to the best you can present.
I know several authors who are represented by Writers House, and with the exception of one, who left before I went with the agency, to a person they all report the same experience I've had.
At one time, probably still, because this system seemed to work well, most agents with Writers House, unless they come from another agency or a position in publishing such as Karen Solem had before she joined the staff for the few years she was there, start out as assistants to the agents on staff (once they all started as assistants to Al Zuckerman, but that may have changed as the agency grew) before moving on to their own client list.
Albert Zuckerman was at one time president of the Society of Authors' Representatives (I think I have that name correct; the predecessor to AAR), and all his agents must adhere to the AAR code of ethics.
On the question of knowing whether your package has arrived or not, there are two ways now to track that. The USPS allows you to place a tracking number on certain packages. This does not require a signature at the other end and is basically unobtrusive to the recipient. The other is just to enclose a stamped addressed postcard with the submission. Mine usually say: "Received your proposal (name of work here) on this ___ day of ______, 20__. Signed ________________________, Writers House."
Even if it is not signed, though with WH my always were, the postmark will tell you when it got there.
I'm sorry I can't answer to the "exclusive" submission part of this discussion. Because of the length of time I've been with them and the nature of my association with them, _all_ my book-length work is exclusive to WH.
Good luck with your agent search.
Mo