Regarding the mouse - the default OSMC skin does not work with a mouse, however if you switch to another skin you will be able to use the mouse.
Which youtube plugin are you using ? There are two different youtube addons for Kodi by different authors, if you do a google search for ‘Youtube Kodi’ you should be able to find both of them and from deduction work out which one you are not already using - give the other one a try.