A good agent can spot the best contract. For the last two years the list price is not what the home is for sale for, it is where the minimum price starts, with sometimes 10 offers 25% above list price.
In this area, the average FSBO bombs 85% of the time, per figures from the Title companies I use. Not Good!
Also, Realtors only get paid after closing, and Lawyers charge by the Minute for assistance, wether it closes or not. And most Lawyers are not competent Real Estate lawyers, as The only Lawyers I dealt with in Real Estate had their real estate Licenses. Which by the way earns them more per year than Being a Lawyer(Lol)
As far as the first question, when a seller accepts an offer from a buyer, both Parties must agree to allow other showings per the contract, in writing.
I just accepted an offer on 21 october from a buyer agent, and I stipulated at that time the home would remain active in the MLS, was allowed to be shown, and backup offers were accepted. They agreed,,,,so the Inspections commenced. Towards the end of the time allowed(seven days)The buyer tried to play a game on repairs, so I Informed him I had another agent that had shown their client the house, for the third time, and were biting at the cuff hoping the first Buyer would walk.
So, I gave the first buyer agent the hard fact: You accept the house, or we walk. They had only hours left, and backed down accepting the property.
So in most cases good agents really earn their money, and Good companies almost always get a higher price than Private owners.
Ask any Appraiser, and he will tell you they will take realtor sales for comps all day long, but they avoid Fsbo's if at all possible, as they are considered unreliable.