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.
I believe the issue is that YouTube is using a new API and the old YouTube add-ons have not been updated (their authors have been absent for a while). I’m using the newer YouTube addon by bromix with OSMC on my RPi
There is a separate issue with RaspbMC that I’ve encountered with a few add-ons where https is not supported. Not sure if that’s relevant here.