I don't understand why your lawyer is telling you that because his business entity is incorporated and yours is a sole proprietorship has anything to do with it.
Last year when I was forming my company I originally wanted to name it Rocky Mountain Home Improvements. There was another company that had been using the name for 3 years but had gone out of business, so I tracked them down and talked to the registered agent (this is a person designated by the corporation to represent the corporation - it happened to be a lawyer who had no financial part of the company) we had a long talk and one thing he warned me was that in their second year of business some loser who operated out of his truck started using their name illegally, the guy was such a loser that he was continually being sued by his customers and suppliers, which of course would end up with the legitimate company being served all the time, this lawyer said he billed them at least 3-4 thousand a year just doing paperwork to prove they were not the same person, that the guy was using their name and that the person suing was suing the wrong people. The company spent thousands of dollars notifying the loser to stop using their name, but since he was just a loser and a beat up truck there was no way to damage him financially as is the usual way to stop somebody from infringing on your name. They too hoped he would eventually drink himself to death or get shot or move away. He out lasted the partners and they went out of business first. This is what he warned me about, that he would work with me to purchase the name from the original partners if I wanted, but warned me that I would be stepping into a mess because I would be dealing with the loser now and his damage to the company name. I passed and did a different name.
If you have been using the name for 11 years you have a great case to show legitimate ownership of it over the infringer, but the moral of the story is, even if you are in the right, can prove it, can get a cease and desist order or a judgement, unless the others are a legitimate ongoing concern, getting any results may or may not happen. But I don't see the business entity having anything to do with it. If you registered the name 11 years ago you should have no problem proving ownership. Trademarking is always advised by a lawyer to help you incase what you are going through ever happens, it helps you prove ownership in court. I would get another lawyer, one that knows his way around this type of case. It is going to cost you some money but unfortunately you won the lottery so to speak and are a victim of these circumstances.