I had a couple of those spring ball things with my speedbar and couldn't figure what they were for! I have been using the Velcro flap at the front of my harness to hold the bar. You may indeed find that fuel calculations based on the gauge might be slightly inaccurate.. Try a measured amount into a drained tank, fly for an hour then empty remains in a measuring jug, or weigh it. You might find it concurs with the gauge - it might be a little different, certainly handy to know for future reference when you might be cutting it fine on a long xc.
Should be ok re pics thanks, I used neo magnets to hold the bar on my last machine which worked well, maybe do the same again.
Cheers
Nige