There is a folder called "songs" in your osu! folder, this folder contains all of your beatmaps. If you have not backed it up before deleting and reinstalling osu!, there is no chance you can get your beatmaps back, you will have to redownload them.
As for maps that you can't redownload as they are "unavailable for download" or just straight up deleted, you might want to try your luck on
bloodcat, they have a lot of songs there that you can't get anymore otherwise.
Regarding your problem "I can't change keys" could you provide any extra information? Can't you find the menu to the keybinds? Does it not accept your input when trying to assign a new key?
I unfortunately can't help you on that one unless I know where it fails.
Also, I cant change my keys.