my general question is, whether this stark contrast in finishing is something people have come across before.
The last house I purchased and renovated had red oak hardwood in not one, not two, not three, but FOUR different stains throughout. The house is 1400 sq ft.
I later learned the prior three owners were artists of varying degree.
I think the solution can be even easier. Most hardware stores will carry uneven transition pieces. They're almost always wood and often are available in a variety of colored stains to match the wood floor. They are beveled in the front, so you don't have any tripping hazard, and they can be easily negotiated by wheeled devices (walkers, etc)
You'll probably ...
I wanted to mention one thorny detail in addition to the other answer. The edge of the hardwood, presumably where it was under the baseboard molding and or drywall, looks quite different from the rest of the wood. This might be because the floor was once refinished by sanding and this section was left out.
Also, good chance that the board ends don't line up ...
Updated based on revisions to the question.
The way I see it you have two options, neither of which is nearly as complicated as your proposed solution.
Install a tile accent row with a beveled or bullnose edge or reducing edger (metal trim). This could be from leftover original tile or something else. 1/2" cement board on the subfloor boards will be ...
No, do not add bridging between open web floor trusses. @Jasen is exactly correct.
Bridging, blocking, etc. does not add much strength for bending anyway. It would be better to add a bearing wall or reduce the span if possible. However, you can’t just add a bearing wall without having the manufacturer verify where it will need to be installed. Then, of ...
consult the engineer.
the bridging transfers loads on the top of the joist to the bottom of the adjacent joists. if the joists are solid lumber this is not a problem, but if they are wooden I beams, the loads could cause the bottom chord to separate from the web. and on trusses could cause the bottom rail to bend