You’ve already got answers about rice so I won’t comment on that.
Once you’ve noted the corroded places, clean it by using 99% isopropyl alcohol and an ESD brush. In lieu of an ESD brush, a regular toothbrush is usually fine but best practice is to adhere to ESD protocols.
The main area you won’t be able to get to is the BGA underneath some of the ICs. To clean under there, the board would need to be put through an ultrasonic cleaner.
I usually give any wet board 24 hours to dry with a fan circulating air over it, flipping the board around the 12 hour mark. Humidity is usually 30-60% where I live.
In terms of storyline, it doesn’t really matter too much.
I still think you should play through the first one. If you need something that feels less 1998, Black Mesa is a remake of the original Half Life and it is very fun.