Hi
First, no one in their right mind would let anyone broker a domain for them without personal references, some successful past sales, and some history in this business.
Too many try to "come-up" on other peoples' names, posing as a broker!
However, to answer some of your questions...
1. Do you recommend requiring the owner to transfer the domain(s) to the broker? If so, how do you deal with the trust issue: owners reluctant to relinquish control of the domain to someone they haven't met?
No, why would I the owner transfer my name to you? duh!
2. How can a broker promote and sell a domain effectively if the owner insists on keeping the domain in his/her account? How would the broker get paid under such an arrangement?
Same way as a realestate agent gets paid...by commission after the sale is closed.
3. Should the broker have exclusive selling rights?
Yes, it would help to avoid confusion as to who/whom is marketing the domain., but it's not mandatory
4. What is a reasonable fee to charge for brokering?
10% > 20% depending on sellers asking price