I think there's more to this question.
a) can the redo/achlogs be modifed by spurious activity from a connected Oracle client? - NO.
b) can the redo/archlogs be modifed by some editor? - YES, they are just files after all. Access to these files should be limited by OS security to admins only (and I suspect that online redo can't be modified in Windows while the DB is up - OS file locking).
c) will logminer detect that an external change has been made to a log? - I don't know. I would hope so (missing SCN's?) - I suspect you'd have to be bloody clever to fool it.
As for the question of changes made NOLOGGING - can someone remind me how that's recorded in the data dictionary? If changes had been made nologging, you would have had problems doing the recovery.
"The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous" - Gibbon, quoted by R.P.Feynman