Of course. Why would a union not want their industrial action to cause the most inconvenience? There would be no point striking on a day when trains aren't running.
Not really justified - shouldn't that be covered by the fare income from all the extra passengers?
That isn't how railway economics work.
Most local/regional trains that are full and standing all day long are loss making, requiring a serious level of subsidy.
Special events make that situation worse as you also need extra rolling stock, cleaning, security, staffing - plus all the risk assessment and other administrative overheads.
You might lose a bit less by charging a higher fare, but from a purely financial perspective, it's still loss making.
It's only when you consider the wider economic and social benefits that you can justify the loss.