"...am I understanding that “excess contribution” is a specific term which applies specifically to situations where you contribute more than the maximum, separate from the last month rule? If so, that wasn’t actually the terminology used by my HSA provider. They basically implied there would be no fees or extra taxes if I switch to a different insurance plan on December 1. "
Your understanding is correct that failing the last month rule's "testing period" does not give rise to an "excess contribution", although it's easy to see why people might say that, since the calculation of the additional income in 2019 is calculating what the excess contribution would have been had you not applied the last-month rule in 2018. But see Section 223 (b)(8)(B) at https://www.law.cornell.edu/uscode/text/26/223 for the actual language; also see under the heading "Testing period" at https://www.irs.gov/publications/p969 , where the $425 is referred to as "income" and the 10% is referred to as "additional tax".
The statement that "there would be no fees or extra taxes if I switch to a different insurance plan on December 1" would have been true only if the date would have been after December 1st (like December 2nd) or if you switched to another HDHP policy.
