I am not seeing what kind of screw that is in the little hole next to the enable/disable switch.. flat or phillips head? and yes, I removed the battery but there was old corrosion there..
Looks pretty bad, but if you are getting a picture from a booting machine, it's cosmetic damage rather than real damage. Lot of surface oxidation but the electrons are still able to travel in the right circuits.
The little adjustment knob is next to the switch on the back, flat head (preferably plastic) and be very gentle, you only need a few degrees rotation to get it right.
You are plugging in the monitor to the 15 pin VGA connector, right? The built in flicker fixer is connected to that, as I recall. The 23 pin connector Amiga video connector still runs at 15KHz no matter which way the switch is plugged.
The best substance I have found for de-oxidising old boards is... vitamin C. Ascorbic acid. A little stronger than acetic acid in vinegar.
Dissolved in boiling water, it eats oxides very effectively, good degreasing too. You do have to rinse it thoroughly afterwards though (more boiling water, then de-ionised water for final rinse).
Then, you can think about isopropyl alcohol. That's the best for final cleansing (it soaks up water) but it doesn't touch oxides and isn't that good at degreasing either.
Leave a couple of days to dry out. You don't need a lot of cleaning liquids, and using cotton buds helps to keep cleaning agents to where they are needed. Even so, a wet board plugged into electricity will cause a helluva lot more damage than a leaky battery.
