Well, that explains everything. Seems you're right.
Funny how neither Serebii, Smogon nor Bulbapedia mention that.
Heh, good call. We should add that to our StrategyDex for completeness purposes, even if it doesn't apply to competitive matches (which it may or may not; I'm not sure).
EDIT: My own testing:
• Mirror Move now works completely differently than it used to. It now allows you to select a target. If the last move the target used was copyable by Mirror Move (regardless of who the move targeted), then the Mirror Move user will attempt to use the move
on the target of Mirror Move. If the copied move isn't a move that allows you to select a target, it targets opponents within range or all Pokemon within range as normal. If the target's last move was not copyable by Mirror Move, Mirror Move fails.
Even if the target of Mirror Move loses a turn due to flinching (and presumably confusion, paralysis, etc.), Mirror Move will attempt to copy the last move it successfully used. Again, if that move is not copyable by Mirror Move, Mirror Move fails.
• I have discovered some important changes to Encore:
1. It lasts fewer turns (this was known)
2. An encored Pokémon can still Shift in Triple Battles. (also known)
3. It now causes the target to only be able to select the move it last successfully used, no matter how many turns ago that was. In 4th gen, Encore would fail if the target's last turn was taken up by a flinch, full paralysis, etc.
4. It now allows the target to choose a target each turn when using the encored move. In 4th gen, the move would target a randomly selected Pokémon from the available foes.
• Magic Guard prevents collateral damage from Flame Burst. (No big surprise there)
• Fake Out can be used successfully if user has spent every previous turn using the Shift function in a Triple Battle.