It depends on the lienholder. Some will only look at one offer at a time, so it doesn't matter if there are offers that come later. Some lenders want to see offers as they come in. Nothing you can do about that. Checking the boxes on 22SS is great, but personally I don't think you can really stop anyone from accepting more offers, regardless of what box is checked. it's the lienholder that controls things, not the seller or the buyer.
Speaking of that box--I always check the boxes that allow seller to accept more offers as well as the box that allows buyer to walk away prior to lienholder approval. Why? You can't stop em anyway. As a listing agent, if I'm telling my client I won't accept more offers on his property, then I'm not really acting in his best interest anymore. And If buyer wants to walk, there's plenty of other contingencies they can use to accomplish that.
As an agent who is currently pending on short sales, both on the listing and selling side, the best advice I can give a buyer of short sales is to make sure your offer is complete, strong, and as close to the bank's minimum as possible. If there's a recent "approved" amount--Don't try to lowball that, unless it's way over the property's value, which sometimes happens. Do your research and be smart--That's your best chance of prevailing. Find an agent with lots of short sale experience--Not necessarily a self-professed short sale "expert" but someone who has a lot of experience. There's a difference.
Also, since I'm on my soapbox--In most cases, you probably don't need to pay extra for a short sale negotiator. Many escrow companies have short sale negotiators and they do this for little or no cost. If your agent has a lot of short sale experience, he or she will already have a relationship with someone like this.