[OuTcAsT]

Does that mean you advocate just letting the thief go?

IMO That would be correct, be a good witness. Unless he is willing to be detained and wait for LE.

That the shopkeeper/clerk should not attempt to apprehend a thief once he gets outside the doors?

That would be an unwise move IMO. The threat has ceased.

If an armed store clerk chases the thief threescore yards down the street and the thief turns and pulls a knife, would you say that clerk could invoke self-defense for shooting the thief?

I would posit that the clerk has then become the aggressor, and it would no longer be self defense but an attempt at vigilante style justice. Once you leave the safety of your shop to go play cops and robbers, (unless, of course, the clerk is LE) how could you claim self defense ? Self defense is for when trouble finds you, not when you go looking for it.