"Dependencies could not be resolved" after swift package update: safe rollback of Package.resolved and build cache

After running swift package update on a Swift 5.9 project, dependency resolution fails with an unsatisfiable-constraints error from the resolver, and I want to return to the previously working dependency graph without guessing at versions.

My understanding is that Package.resolved records the exact pinned versions, so restoring the committed copy of that file and re-resolving should reproduce the old graph. What is less clear to me is how much of the local state actually participates in that rollback: the .build directory still contains checkouts and artifacts from the failed update, and I am unsure whether a plain resolve reuses or ignores that state, or whether swift package reset is required to guarantee a clean recovery.

There is a second uncertainty: this project is also built with a newer toolchain in CI than on my machine, and I have seen claims that build caches are not guaranteed to stay valid across toolchain upgrades, though I cannot find this stated as a documented invariant rather than convention.

Specifically:

1. Is restoring the prior Package.resolved plus swift package resolve sufficient to roll back a failed update, or can stale .build checkouts silently affect the result?
2. When does swift package reset change the outcome compared to resolve alone?
3. Is cache invalidation across toolchain upgrades documented anywhere, or only community practice?