First, participation in MLS is not required of a real estate broker, although with how it helps get homes sold, he's not really serving his clients to the best of his ability, in my opinion.
Since it is not in MLS, he does not have to allow other brokers to show the home nor does he have to pay them a cooperating commission if they sell the house. So, the brokers from the other agencies see that they won't get paid, they won't want to show the house, much less sell it. Where this gets worse is that many agents and buyers work together under "Buyer Agency". Under this there are many benefits to the buyers, but one protection to the agent representing them is that if they buy a house while under contract to him, they owe him a commission. If that commission doesn't come from the listing broker, the buyer has to pay it. Not many buyers would want to pay that extra money.
Where you might have an argument here is that under Wisconsin law, a licensed agent or broker is required to put the needs of his clients ahead of his own. You would need to consult an attorney here, but on its face, by excluding other agents he is trying to make the most money for himself while sacrificing a quicker sale for you.
His advertising is his advertising. There is no regulation on it other than "to the best of his abilities". If he has found his email blasts work, and for some it does quite well, then it covers that requirement.
By "6 month trial" I am assuming you mean his listing contract with you lasts for 6 months after which time you no longer have to work with him. That is standard. All contracts must have a beginning and an ending date. However, the claiming of full commission after cancellation is kind of odd.
This is where you need to check your listing contract. If it is a standard Wisconsin contract, then he would have to hand-write (or type in) the clause that says he gets paid no matter what. If this fee is a percentage of the sale price, then how can he determine what to charge you if the house doesn't sell? He can't. If this is a flat fee, then he is likely entitled to be paid if that is what you agreed to with your signature on the contract. Again, this is attorney territory here.
What it sounds like to me, is that he is taking full advantage of his placement on a small island where he basically thinks he has a trapped clientele. What you need to know is that any agent from anywhere in the state could list your home for sale and they'd likely list it in MLS and have other agencies show it. This is usually not common practice since an agent like me, who lives in Mukwonago, has pretty much no idea of the market in, say Wausau. But, you could contact an agent "on shore" to list for you. The current broker cannot block others from listing on the island.
I wish you the best of luck.
If it is located in Wisconsin you can contact the state and file a complaint. They tend to act quicker when a consumer files a complaint.
The MLS is an offering of compensation to cooperating brokers. Nothing more. This broker does not appear to be offering compensation to MLS participating brokers. If this is the business model and their marketing efforts are clearly displayed on the listing with the seller then they do not appear to be doing anything wrong.