run from the DI to your PA.. EQ your tone at the mixer for FOH.. use the 210 on stage as a monitor, EQ on your head to the tone you like, but try to get a general match between the stage and the PA.. if you have a stage monitor, run through the monitor to help spread out the overall sound.. try to position your cab near a back wall, or preferably in a corner.. if you have a way of tilting the cab up, do that so that the sound isn't ankle level.. or if you have a combo amp stand, use that..
no, the DI is not going to draw any power from your head.. but your output will be less than using the 15 with the 210..
if the gig is in a small bar, you should be fine with just the 210..
man.. 15 cover, all you can eat wings/bud light.. that sounds really good..
good luck with the gig..