past the triggering block. A block-chain re-org of two thousand or
more blocks on the main Bitcoin chain is unthinkable-- the economic
chaos would be massive, and the reaction to such a drastic (and
extremely unlikely) event would certainly be a hastily imposed
checkpoint to get everybody back onto the chain that everybody was
using for economic transactions.

No, the "triggering block" you mentioned is NOT where the hardfork starts. Using BIP101 as an example, the hardfork starts when the first >1MB is mined. For people who failed to upgrade, the "grace period" is always zero, which is the moment they realize a hardfork.