Can corroded terminals cause occasional power flickering in house?
Yes, that is a very serious situation.  You have a breaker in direct weather that isn't made for that service, and worse, you have aluminum wiring in and out, which is vulnerable to corrosion from weather.  That needs to be looked at every year or two to clean up the wires and lugs and refresh the No-Ox.  

Yes you can de-energize it.   You need to pull the electric meter that is before it.  

However, when you call up the electric company to have them do this, they will probably inform you that you have a smart meter and they can shut it off remotely by typing a few things on a keyboard.   So do that. 

And then, do something about all that gear being exposed to weather, because that is the root of the problem.