I would be very interested to see one of the unedited images to see exactly the situation you were up against.

You have two separate problems here, flare from the sun/light in front of your camera and a heavily backlit subject.

If your positioning is restricted behind the finish line to get the light source more out of frame are you able to consider getting up higher and shooting down? The problem with finish lines is that the runners can come on the left, mid or right side and one shooting position is never ideal. There's always someone else where you need to get and if you do get where you want along comes a camera phone on legs !

Having shot thousands of runners often in harsh sun and shooting into the light I don't recall ever having a problem with glare affecting the runner. A good lens hood and high quality filter (if any) will help and they also say you can add additional shielding from the light with your hand but I have never done that.

As for the backlit subjects I recently trialed the use of TTL fill flash in HSS during a marathon where many shots were into the bright light and with dark skinned subjects you get problems. With mirrorless this is a doddle being able to quickly adjust for ambient light using the histogram and the flash looking after the subject reasonably well.

If they demand a w9, you could potentially be liable for taxes on this income. My son is a professional musician and has often played gigs requiring a w9, and as a result he almost always has had to declare these gigs as income. Depending on you normal income bracket that could be a healthy chunk of your $50. They don't need to report anything under $600, but they can. Over the years my son has had to pay taxes on amounts that were under the minimum reporting amount because the venue reported it anyway. I would say thanks, but no thanks unless they give you more money.