[...]
I had confused what's network-adjusted time. Bitcoin Wiki clears it up:
"Network-adjusted time" is the median of the timestamps returned by all nodes connected to you. As a result block timestamps are not exactly accurate, and they do not need to be. Block times are accurate only to within an hour or two.
So even if a miner tried to alter it, the full nodes that would relay it, would reject it. As for the median timestamp of previous 11 blocks, isn't it the timestamp of the 6th from the last 11 blocks?
It would drop to 25%. It would drop by 75%.
Right. That sounds more reasonable.