If you reduce your 10.5 GB file to around 8.5 GB to fit on a DL disc, you'll be reducing it to about 81% of its original size. That shouldn't affect its quality too much. As you pointed out, some of your intended recipients won't be able to read a BD disc so do you really have another viable option?
What are you planning to give them, a video file simply stored on a DVD, or a proper Video DVD?
I don't quite follow your last question. What would you be doing with the file in Video Copy & Convert? You're going to have to either chop 20% off in an editor to cut it down to size, or re-render it at a different size or bitrate to fit on the disc.