Also part of this issue: Opening the Command-Block GUI freezes the game, but not the music. When exiting, the music often repeats
Resolved as of 14w26b
What exactly is the use of this 'feature'? If an op flagged a mob to be invulnerable, why on earth should it be vulnerable to some players? It still can be killed with /kill
The c parameter LIMITS the selected players to the given amount. It still works if there are more players. try /say @e[c=3] It will output 3 Entities even though there are more in your world. You can use the comparator output strength to count the players.
Do you have particles enabled under your video settings?
Also if you pass the invisibility directly with the summon command, all mobs become translucent. Command:/summon Creeper ~ ~1 ~ {powered:1,ActiveEffects:[{Id:14,Duration:2000000}]}
I doubt that this is intentional since the music does pause when pausing the game through the game menu