Rinse with tap water, then pool water, but I don't think it makes a difference. Not sure if it would, I've done both and never had issues with varying results.
Yes, if your CYA level is 30ish you can easily lose 50% of the FC during the day. So if your FC is at 12 in the morning it can be down to 5 or 6 by nightfall when you are shocking. So usually towards the end of a shock process when the FC drops are smaller, we recommend shocking in the evening to cut back on how much is lost to sunlight.
I think you are overthinking things... try some POP. The test is fairly simple to do, it doesn't matter if its one scoop or two, level or not, or if all the powder dissolves. All that matters if it it turns pink, and how many drops it takes to get it to clear. Try going very slowly, perhaps have someone there with you to verify the drop count - make sure you aren't adding multiple drops at once, etc. make sure the solution is staying clear for 15-30 full seconds before moving onto the CC portion.
Try doing the overnight test without adding any bleach. So say you haven't added any in a couple of hours - go test, it should have had time to circulate well. Don't add more bleach, just write down the result. Then tomorrow early, before the sun hits the pool, test again. Leave the filter running all night.
I'm sorry if I'm repeating things you know, I just want to make sure we aren't missing anything.