For the ‘line in use – no’ thing, it was said before, said vehicles to depot then assign them to the line again and in a few seconds it should turn to Yes.
For the above scenario what I would do:
1) coal-ore-steel mill line
2) steel mill – city line
a couple / few carriages each, enough to be faster than walking (ore will stop walking will go to station instead)
Also I don’t set wait for full, sometimes it’s too slow to fill a car and frequency goes up way too much. Just always make sure you use just as many vehicles you need to have good enough frequency (so line works) and cargo doesn’t pile up in station a lot.