My first step would be to try a little lubrication / deoxidation to try and loosen up those switches and restore operation. If you have some Deoxit D5, you could try a light application of that to the stem of the switch (remove the cap first), and work the switch, a lot. Repeat if you think necessary, but don't use too much.
Some people might wince at using WD-40 but I've used it before and it seems to work okay - but a product like Deoxit is formulated specifically for this kind of application (deoxidation, lubrication).
It's possible there is something more that just corrosion preventing the switches from working properly, but these switches can be taken apart and cleaned / repaired - if you're remotely comfortable with a soldering iron and can do fine detail work. Here's what they look like apart:
https://deskthority.net/wiki/Alps_SKCC_Tall_Cream
