have you watched spartacus - the australian tv show. Low key spoiler - In the ludus (training school of the gladiators), they do keep yelling "doctore" at their teacher. Docotore is latin for teacher and yes, that is where we got the idea of calling phd's and medics doctors). So maybe they did the same in Scotland at some point? Who knows. It is not like Duo ever EXPLAINS anything. Leaves us in the dark to figure it all out on our own. Duo is not a very good teacher :(
I asked that question once before. Far as I can tell both versions are pronounced the same. Tidsear is the root form. Thidsear is lenited but not slenderized Thidseir is lenited and slenderized.
So far, all forms I've seen that call for lenition also call for slenderizing. I cannot say if this will always be so.