I have no idea about overhead for engineering because I am a 1-man show and work out of the house. I do know the home inspection costs are mainly your vehicle and insurance. The tools and equipment needed for a professional inspection business are far less than the tools and equipment for an engineering business. You can run an inspection business out of your house because you go to the work, it does not come to you. Engineering requires constantly updating codes, a lot of software purchases and rental of some software. There is also a constant learning curve that is tougher on a 1-man show. Large companies have a masonry guru, a cold-formed guru, concrete guru etc while a 1-man show requires you to be all of them or don't do that type of work. That constant learning curve pays 0$/hr. That is why I said wanting a company with employees may be profitable while starting a new 1-man show may not be by the time you obligate 8-9 years to education and training. And when you are done in college, you need to learn masonry, cold-formed steel, concrete tanks and silos, steel tanks and silos, aluminum and all the other avenues not covered in college. Buying a $1500 plotter and sharing with 4 engineers is cheaper than buying one for yourself only.
I don't know what the engineering rates are where you are at, but where I live, I don't think the engineer is grossing $100/hr. The company is probably charging an average of $170/hr for the engineer. Your problem will be the fact you are the Owner and Employee. Most companies do have a 3 multiplier. Included in the multiplier is the cost of the head cheese. In many companies, they do not do a lot of billable work. Again, easier to portion out with 4 employees than 1. Call around and get some ideas of their hourly rate for a structural engineer not just an engineer. Some jobs pay better than others. A structural engineer in one company in a given town may make a lot less or more than another. What matters is where you live. Also, just getting your PE may not pay the same as 20 years with a specialty ability that people are willing to pay for.
I noted you do expert testimony. That generally pays better in most fields if you are good at it. I also noted you said you work for the inspectors. I would not limit myself to one side of law suits. I do know of several home inspectors that needed to be sued. They advertised they would "find everything" wrong with a house, got the job and did a really poor inspection. I know, I have read their reports. Just one example, a carbon monoxide pipe discharging into the attic of a house that also had attic bedrooms. "Need to be concerned about it possibly adding moisture in the attic". I reviewed the report for a some structural issues they missed, but instantly noticed the flue pipe picture and comment. They did not advise the discharge be corrected.
