I tried to replace a cracked sillcock last weekend but the main shutoff
didn't work. I turned it all the way off, but water would still trickle
through the faucets in a very thin stream.

I'm guessing that this situation will make in impossible to replace the
sillcock? Or are there any acceptable solutions I can use? It has a
male threaded end, and there is a 1/2" copper supply pipe that i would
have to cut.

Fixing the shutoff right now will be quite a task as it is located
about 2-3 ft underground. There is about a 6" pipe from above ground
dug in to access the valve. Will a pro be able to work within these
constraints to replace the valve? I can't imagine that one could, so
I'm resigned to ripping my landscaping to dig a hole. Also when i
get around to doing this, does anyone have any suggestions for somehow
redoing the shutoff so that I can get at it better?

Thanks!

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XBRG