I am an architect, licensed for more than 30 years.
What your chances are in court will depend largely on two things:
- What your contract with the architect says about his duties and responsibilities; and
- How competent and responsible other architects in your area are.
Without having any detailed knowledge of either of these variables, I'll say that -- in general -- an architect has a duty to design and to provide construction documents that comply with the applicable building and fire codes. We are not supposed to "assume" things where the presence or absence of those things may affect code compliance.
I'm not sure how a truss or trusses on top of a brick fire wall maintains the continuity of the fire wall. A fire wall is supposed to be continuous from the foundation through the roof, or to the underside of the roof deck IF the roof deck is non-combustible for 4 feet on either side of the fire wall. If the intent is to transition from a brick masonry fire wall to a wood stud and sheetrock assembly through the attic level, whatever the wood stud and sheetrock assembly is must be something that has been tested by an accredited testing laboratory to prove that it can withstand a fire for however long the rating needs to be (1-hour, 2-hours, 3-hours).
Since the architect didn't address this condition in his construction documents, most likely he is now liable for whatever it costs to make the corrections -- sort of. More precisely, if he had shown it correctly you would have had to pay for it, so you should pay to install it later. HOWEVER, as you note, it's more difficult (and thus more expensive) to do it now than it would have been at the time the work was being done and access wasn't as difficult. So what the architect is actually liable for is the difference between what it should have cost and what it's going to cost now. You said it's going to cost you $4,000. If it would have cost $1,500 if performed at the logical stage of the construction, the architect is only liable for the difference of $2,500.
Most architects carry professional liability insurance. Unless your contract with the architect has some strange, non-standard language in it, typically architects (and engineers) are required to perform according to the usual and customary standard of care prevailing in your area. What that means is, his performance is compared to what other qualified architects in your area would have done in the same situation. In court, this often becomes a matter of dueling experts. Your attorney will bring in an architect or two or three who will testify that any reasonable architect would have verified the condition and the extent of the brick fire wall. The architect's attorney will bring in other architects who will testify that any reasonable architect would have assumed that the brick fire wall extended up through the attic. In the end, it will come down to whose experts the judge or the jury thinks are more credible witnesses.
Professional liability insurance is written based on the ordinary standard of care -- which, as I wrote above, is based on what other reasonable architects in your area would do in the same situation. What this means for you, unfortunately, is that even though it seems crystal clear that your architect messed up, if other architects in and around Detroit agree that they would have made the same assumption -- you probably won't recover anything in court.
He can -- and probably will. If your lawyer is any good, that won't fly in court,. The code requires that construction documents show -- "in detail" -- that the proposed work will comply with code requirements. That requirement applies whether or not the plan reviewer notices any problems. The plan reviewer is supposed to question anything that appears incorrect or incomplete, but the fact that a plan reviewer missed something as important as the continuity of a fire wall does not absolve the architect of the responsibility to produce drawings that will result in compliant construction.
Unfortunately, yes. The ultimate responsibility for having a compliant building falls on the owner. In the real world, the owner typically is neither an architect nor a builder, so owners delegate the responsibility first to an architect (for the design and the construction documents) and then to a contractor to build the thing. It's a pass-through.