1. That's likely to be one drain valve on the flow from the boiler and one on the return to the boiler. As stated by SimonG, opening either one will drain everything in the system above the valve. Opening both will drain it all quicker.
2. They are quite likely to be the only drain valves on the system.
3. Why not drain down the whole system, then close both radiator valves, and the only residual water you should have left is any in the pipes from the radiator to the drain off valves. Should be none, but you never know! Slacken off connections slowly, ready to re-tighten if any leaks. Plenty of old towels over the carpet.
4. With valves closed, remove radiator. Disconnect at pipes, not where radiator valves go into radiator.
5. Swap valves or put new ones on new radiator.
6. Install new radiator, connect and re-fill system. When you are sure there are no leaks, shut off feed to small tank in loft, drain until small tank is empty, drain another two buckets full, add inhibitor (e.g. Sentinel X100) and re-fill.
Note that if your new radiator has pipe centres which are further off the wall than the existing pipe centres, you are going to have to modify the pipework. May not be easy with boxing in.
While it would be nice to replace the drain off valves, you might be able to unscrew the body and fit new ones, such as BES part number 6920 (
Drain Cock | Brassware | Plumbing Supplies | BES.co.uk ). If you do, clean off all that green stuff with a scouring pad or similar. It looks like flux residue, and while it has probably done all the corroding its going to, why take a risk?
