Let me see, you are assuming the distribution grid is not functioning, but you still want to start up those generating plants to do what exactly?
You also need to add load in a coordinated fashion, or you aren't going to be able to ramp up those plants anyway. The reason they shut down in the first place would likely have been due to loss of load. It's like stepping on the accelerator in a car when you don't have the clutch engaged. The generators would over-rev and have to shut down to protect themselves.
A major cause of delay in restarting service is that with thermal plants that tripped unexpectedly instead of going through a controlled shutdown the turbine (and possibly other systems) would have to go through a cool down and restart cycle before it can be restarted without damage. That can take several days. This is why a major blackout can last for a week or so before everything is back on line.
So assuming the entire grid tripped, the start up sequence would go a) start up the plants that have black start capability, b) ramp them up while adding load in a controlled manner, c) use the power from them to start up other plants, d) add more load incrementally, e) ramp up the power and load together in a coordinated fashion until everything is back to normal capacity. It's not like turning the key on a car, it takes time.
I think you've perhaps missed my point, which is not really about power plants at all.
My point is that protection of CNI is a key priority, and "all" a hostile agency has to do to mount a major attack on a foreign nation is take control of their power grid. I don't want to ramp up any plants, I don't want to thinking about sequencing load and all those good things. Those are just consequential problems and aggravating factors in the attack.
All I wanted was to point out that if a hostile agency seized control of the grid, they could seriously disrupt an entire country for an extended period of time, precisely because of the reasons you quote. Hence considerable effort is expended trying to defend against attack.