I am looking to use the 1/4" stereo headphone out jack on my Crate GLX15 amp to patch to a line out to a mixer board. Do I have to do some signal conversion to match the normal line out signal? Or, can this be done with a simple "Y" cable to convert from Stereo to Mono? I am hoping that I can still use the speaker out function on the amp to drive a small cabinet so I can use the amp's output for a monitor at the same time. Can this be done?

The short answer is no. Impedance mismatch, resulting in distortion will be the result.

Why not simply mic the cabinet, and capture that sound? If you have to have a DI, then take it after your pedals and before the amp input, but don't expect it to sound anything like the amps speaker.