I agree with everything you just wrote. But, you have not addressed
my question. In what way does/can following the glideslope from 2000
to 1800 feet at SCK violate the regulatory implications of the SCK
ILS's 8260-3?

There is no rule that says "thou shalt not follow the G/S unless it
is primary". What the rules say is "thou shall not bust the published
altitude restrictions prior to the PFAF".

Since it isn't logically possible to violate the altitude
restrictions *in this instance* by following the G/S, it can't be
illegal to do so.

The regulatory basis is 91.175(a) which requires "Unless otherwise
authorized by the Administrator, when an instrument letdown to a civil
airport is necessary, each person operating an aircraft, except a
military aircraft of the United States, shall use a standard instrument
approach procedure prescribed for the airport in part 97 of this
chapter."

Part 97 does not prescribe pilot technique. It prescribes the tracks
and altitudes to be flown. If those are complied with, there's no
possible violation.