Why 'leibh' then 'thu'? Shouldn't they be the same?
Technically. B' aill leibh is a petrified phrase. You could (and grammatically should) say b' aill leat. But, no one does regardless of the formality.
That is, a bheil sibh ....
No, as noted above, it's a fixed formula that's always plural/formal, even when you would otherwise use singular you.
I put down the exactly the same sentence!!!!