You should install any fitting so that the pipe gets bottomed out in the socket. Now, will it be strong enough and water tight if it's not? Usually, yes, but it's hard to get solder there...you may need to add solder from the other side of the coupler, but that is tougher to get everything done properly and then get the other end soldered, too. If the pipe is in contact with the concrete, that will act like a big heat sink, and it will be really tough to get the pipe up to temp to ensure the solder will flow properly. It would be easy to end up with a cold solder joint. Since you won't be able to really see the other end well, it will be hard to get a good visual inspection of the newly soldered joint.
A compression fitting also must be bottomed out in the socket, and 1/2" is probably too little sticking out as the compression nut will stick out more until it is tightened, preventing you from getting the valve bottomed, and once you start to tighten it, you may not be able to move it to eventually get it there.
What I might try is to get a diamond hole saw a bit bigger than the pipe and use it to make a cut around the pipe. You should then be able to probably chip out the cement around it.
If it really is only 1/2", something like a Sharkbite won't work.
You may need to repair from outside and stick new stuff through the wall.
