Minimal corrosion from a drop of water has caused LB damage. Why?

So on Tuesday, I touched my trackpad with wet hands. I was in a rush to go to school so I wiped the drop of water on my trackpad. Approximately 3 and a half hours later I'm working on a project with my schoolmate, so I take out my laptop. The trackpad is extremely fatigued and slow. Eventually I restart my computer and an animation of my trackpad not working plays. Even worse, suddenly the keyboard fails. Fast forward to Sunday, the 31st of October. I take in my laptop to the apple store to diagnose the issue. Turns out I missed out on my warranty by a month. The apple tech than takes my laptop to the back to take the back off. He returns with my mac with the back off and shows me some water damage under the trackpad. The corrosion is very minimal (I'll attach some images). I talk to the manager and he says corrosion would set in after 2 days. Yet my laptop stopped working within a few hours, meaning the corrosion must have set in much quicker, which he claims is impossible and there "must have been water settled there before" even though my laptops first contact with water is what i mentioned above. Anyways the cost is a flat fee of "788 dollars" to replace the whole logic board. Smh

So why was corrosion set in so quickly?

Secondly, how did a small drop of water on my trackpad (that I quickly wiped away), cause corrosive damage?

Lastly, what is the cheapest possible way that I can repair my laptop? Is there anyway to manually rid the mac of this corrosion? Can I myself do it? Would a third party be able to do this cheaper?

-I only realized afterwards when the tech told me that there was water damage that me using the track pad with wet hands supposedly caused the water damage. I would have never assumed that such a small amount of water have done this. I mean, hasn't everyone drank water, while on there laptop and some spews out of there mouth accidentally and very minimally on there laptop? Or they are eating food and they get there keyboard a little oily? I mean these are natural things that I would never assume could cause damage. I've never dropped my laptop, spilled any drinks on it, and it is always covered. I never take it out in the rain if it does, and i have a pretty nice bag for it as well. It bothers me, because of a little mistake supposedly my laptop is done for. Even worse I need it for school -____-.

Hopefully someone can help me!

Thanks