Move a coffin and it’s corpse

Chances are very good that you built the jeweler's workshop in such a way as to block the only exit. This is very common with the jeweler's workshop because (I think) it's the only workshop that blocks one entire side of the 3x3 grid (the right or east side). When you build the workshop, the darker X's show you which tiles will become impassible. If this is the case, deconstruct the workshop and build it elsewhere.

Otherwise check for forbidden doors or defined burrows that don't allow access.

To answer your first question, yes, all creatures can move diagonally.

Your underlying problem is with the order of the designations. Dwarves operate on a Last in First Out system (LIFO) which means that for squares, if you, intuitively, mark all four walls at once, you'll get one corner (if you went clockwise) or a variable amount (if you went horizontal -> vertical or vice versa) that can still be less then four.

Once this has happened, you must deconstruct walls to reach the corners, as construction can't happen diagonally.

Prevention of this is actually pretty easy as long as you're aware of it: simply build corners first, either by not assigning other construction jobs until the corners are complete, or by suspending one or more adjacent wall constructions until the corners are complete, then unsuspending them.