Not sure Cat what you mean but "tube skinned" but if you mean cased, and like a muskrat, that's different.
We hooped them and slit them from blowhole to nose and rolled them as we skinned in what would look like a Christmas Manger contraption.
Easy? April Fools! I've rough skinned them kneeling in snow banks in MT many years ago when young, and would dull 2 good skinning knives on a beaver!!
AS for the rest of your questions on skulls...there is/was a recent thread on 'tried a few skulls" or something like that where the chap outlined a method he used and others chimed in. Check your search function and see if you can locate it.
I have teeth that easily slid out when the skull dried... dang...they go forever up in their head!