Interesting conversation. Although the first video was scary to watch, I watched it again and did notice the first house wasn't even completed which to me leads to the interview to be inconclusive. You can't compare a finished home to a new home under construction with just wood framing and then say the new home burned down, it just doesn't make sense or maybe I overlooked something. I don't have my house covered except for windows and roof but I do know it would burn down faster than my neighbours house which is 60+ years old because my home is essentially *naked* and I would have nothing left if both our homes were to have a fire. As for insurance, my insurance company hasn't asked anything about our trusses vs. traditional lumber so my premiums would remain the same. I was pretty concerned about this whole thing after reading it yesterday and called our city inspector (who's also a friend) and asked about the pros/cons and his response was "if new homes have this problem which he hadn't heard of, older homes have others you may not have such as old electrical issues so there's concerns about everything causing a fire in any type of home". Not trying to argue with anyone but essentially everything in our homes are a fire hazard that can bring down a house in a matter of minutes. As long as everyone is out safely, that's what we all have insurance for.

"Does anyone have experience with using steel beams to support sagging joists?" Yes. They're a great way to accomplish load bearing in a small package vs. wood. Your GC MAY have all the experience and expertise required to accomplish your minimally described situation.... But, I wouldn't support the beams without additional footers (as pointed out by Brickeyee) without seeing the existing structure, which may permit it. I assume you mean rectangular tube, two webs vs. one for I or H beams. Unless the former has price advantage, I'd use the latter. Leaving aside AHJ and engineering issues, why doesn't the GC address the bathroom floor alone?

Flitch plates are normally used between wood members (and through bolted repeatedly) to provide the plate with buckling resistance. The metal is normally relatively thin compared to its height. By trapping it between wood members you prevent buckling, but it needs enough fasteners to not exceed the crush rating of the wood in two planes. The structural load is vertical (parallel to the height of the beam) and the buckling load horizontal (perpendicular to the face of the beam). You could use thicker metal but it would be far more effective to either use u-channel or angle iron across the damaged area. I would use either U or angle on both sides with though fasteners. The leg of the angle perpendicular to the side of the beam acts to prevent buckling and the metal on each side distributes the load across the face of the beam and prevents crushing under the fasteners.

I'm assuming you are talking about a beam to support the joists of the 2nd floor. There is no code issue with installing the beam into the floor system of the 2nd floor. We do it all the time with both LVLs and steel I beams. The structural engineer will need to size the header and load points as well as make any adjustments to the foundation or footings. What we do is build temp wall to either side of the existing load bearing wall. We then remove the wall and cut out the joists for the width of the beam to be installed. You can then raise the beam into place and hanger the existing joists onto it. Install your load points, and increase foundation as needed before removing the temp walls. If the engineer sizes an LVL beam that will be taller than the floor system go to a steel W8 x whatever is needed to ensure it fits within the 2x10 floor system. Pack out the beam with 2x material and through bolt it. This will give you something to nail your hangers to. Code does not allow anything to be drilled into the beam so plumbing and electrical will need to be rerouted. In this ceiling we have a steel beam with two sets of LVLs attached to it and the floor joists so we could remove off set load bearing walls in this 1950s home. It's carrying a story and a half above. It can be done!