Just ask the PB or the stone mason that did the work if they think you should seal it. I have unsealed flagstone all around my pool but none of it sees continuos water flow, just splash out. There's always a big debate here on threads about sealing versus not sealing when it comes to soft stone like flagstone or travertine. I personally like the color of my FS and I don't want to go through the hassle of sealing it all. So I'm willing to trade off a shorter life span for a more natural look and feel. To each his own. But, for a flowing water surface like what you have, I'm not so sure I would be so cavalier. Just ask, no harm in asking.
Yes, so when Friday rolls around, start bringing your pH down. It's probably going to bounce right back up again, but just keep bringing it down. As you're doing that, adjust your FC to shock level based on a current CYA reading. Use the FC/CYA ratio chart in pool school to determine your shock FC level and use Pool Math to get your dosing information. Then when the sun goes down, get it set to shock level and then remeasure the following morning. Make sure your pumps are running night long. If you lose 1ppm or less and your CC's are less than 0.5ppm, then you're all good.
Your last chore will be the best one - GO SWIMMING
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If you want to run a full battery of tests and post the results before you start your OCLT, feel free. It would be helpful to know where your water started.