the list goes on. Question everything
Indeed, and make an effort to understand everything in you plane. For a Part 23 airplane there is no requirement to account for a control cable break from a handling/redundancy standpoint. Part 25 has differences in this regard. Very simply, as DB has remarked, very effective flaps, if asymmetric, could not be overcome with aileron control. In the context of a Cessna, this becomes even more extreme for those Cessnas modified with drooping ailerons connected to the flap system.
The rule about interconnection has it's roots in hydraulic flap systems. Before electric motors were common, and when the flap actuation forces were very high, a hydraulic cylinder was used to operate each of the flaps, which had the effect of making the flaps independent. Thus, a separate interconnection of the two flaps became desirable/necessary. So, in theory, that interconnection could fail, though both flaps still operate = problem. Most GA airplanes with more recent designs (all 100 series Cessnas) have flaps which are cable operated to move interconnected as both the actuation, and interconnection for both flaps. You can disconnect the Cessna flap motor, and the flaps will still move together. Thus, a failure to operate together is much more a risk than asymmetry. Indeed, due to the excellent aerodynamics of Cessna "paralift" flaps, their operating forces are happily low. Fly a manual flap Cessna, and you'll feel how nicely they move.
As I pick back through earlier versions of Part 23, I find:
Sec. 23.701
Flap interconnection.
(a) The motion of flaps on opposite sides of the plane of symmetry must be synchronized by a mechanical interconnection unless the airplane has safe flight characteristics with the flaps retracted on one side and extended on the other.
So I do see where the reference to flight with flap asymmetry has a historical origin. Though an applicant for a modification to a presently certified plane could apply to use this standard, it is doubtful that it would be permitted. The FAA "Changed Product Rule" (CPR) would be invoked by the regulator, and drive the applicant to use a later standard. A new design would be required to use the most recent certification standard.
If an element of an aircraft design is found to be unsafe, a service bulletin (SB), and perhaps an airworthiness directive (AD) will be issued to correct it (SB sometimes optional, AD mandatory in all cases). Imperfect aircraft designs may remain in service as is in the absence of an SB/AD, even though we would not do it that way ever again. If that system becomes the subject of a modification, CPR will be considered to invoke a more recent certification standard if appropriate.
When I develop certification plans (for STCs I issue), one of my duties is to go through the CPR process, and either determine (if it's within my delegation) or recommend to the regulator (if is exceeds my delegation) which amendment of the design requirements should be applied to the modification. Often, I just use the most recent amendment of the standard for convenience. The only problem with doing that may be that other aspects of the older aircraft could not pass the newest standard (seats, for example), so you'd fail before you begin.
An applicant can always propose to make an aircraft much more compliant that the requirement states. It's usually expensive and heavy, but sometimes a good idea. The challenge with dual interconnections of control systems can be that the dual systems fight each other if misrigged, and create undesired friction, or very high forces within the interconnected circuit, and break something.
I have issued approvals for modification of flap systems, though the operating philosophy was not affected, so the original design standard was still appropriate. I have no concern about Cessna flap control systems which are correctly maintained, and used.
Scifi, pulling the breaker is excellent - except that most Cessnas have the cheap CB's which you cannot pull, so you're stuck with the master being the only control you have over that circuit. I replaced the CB's in my 150 to be the type you can pull - for that very reason!