I haven't looked through the entire thread. Has anyone mentioned 91.177?
Once you achieve an en route altitude 91.177 is constant and mandatory except
for a Part 97 IAP or special IAP. (excepting a visual, contact, or
cancellation)

Roy Smith wrote:

Peter wrote:
One could read 175 as saying that if an official IAP is provided, it
must be used.

I think one would need to be reading a different text than what I have
available to come to that conclusion.

The one I've got says, "each person [...] shall use a standard instrument
approach procedure prescribed for the airport in part 97 of this chapter."
I don't see anything in there that sounds like, "if one is provided".