I think one of the biggest misconception is that John Q Public has, is that they expect a home inspector is a certified Engineer and the report is as such as well. They are not and if anyone thinks for $300 or even $500 your are going to get a certified engineers report on your home you are sadly mistaken.
As far as an open building permit on the deck is concerned, I think you have case on the previous owner not the home inspector. The home inspector does not check to ensure that all is to code or with the town to see if anything is outstanding. In fact I think it is the lawyer that should be checking title, survey and other open issues with the town such as open building permits. The lawyer should have caught this and should help you out for free.
I had a storm sewer issue on a home we bought, the previous owner cut corners when they connected and our lawyer went to town on them. Took them two years to pay us, but they paid us.
The problem with the industry is there is no requirements in Ontario to certified these folks. Today you can work on the line at Ford motor company, tomorrow they close the factory and you are a home inspector. No training, no certification or insurance required, just hang the shingle so to speak.