Can I use the Nikon ML-L3 Remote Control to take 10-15 minute exposures with my Nikon D5200? If yes, then must I keep the remote pressed during that entire period, or can I lock it somehow and press it again to stop the exposure? Need help.
Yes you can, in full manual mode on the D5200 you can use Time mode which is a shutter setting in full Manual mode to do so. You will need to click the button once to open it and again to close the shutter. The full reference manual even suggests using the ML-L3 specifically (page 60).