Hello 24, the issue may be a cal/mag deficiency, the leaves are exhibiting signs of both, but that could be due to not enough cal/mag or a ph issue that is preventing its absorption. But, 10" sounds very close to me! I'd go with 20" but if you can't raise your lights that far then I'd do something I hate to do but have had great success with..if you do it right I think its more dramatic for us than for the plants!
*Check out the tutorial here on GWE on supper cropping_ basically you carefully pinch the stem until you feel it "crush", then carefully bend the branch over sideways and tape the joint/bend with some tape_ the kind made for grafting plants is probably the best but I've used sports tape very successfully too. This will enable you to get your tallest colas more level with the ones below and give you a little more "head space". Super cropping always makes me nervous but its always worked out well. If your tallest colas are a little more advanced than the lower ones then it will give the lower ones time to catch up and even out your harvest as well. Also_ if the cola bends over more than 90 degrees then you can help support it with some string or plant ties. Another way the worked really well for me was to tape an "L" bracket to the stem right below the bend and then tape the bent stem on the top side of the bracket_ giving it good support without the ties.
Good luck and I hope it all turns out well!
PS: how cool is your grown room getting when the lights are off? The purple color might also be due in part to temps lower than 60 degrees F.