Mashiach agent-
You are wrong.
When 10 teves falls on friday, we DO fast on that day.
Likewise if erev pesach falls on friday, the taanis bechorim is on friday.
And just to get the terminology straight – it is only a nidche if pushed off i.e. Shabbos to sunday. If it is pushed earlier (as taanis esther sometimes is) its called a mukdam