Listerine is friggin' useless. It only kills bacteria for a few seconds and then it grows right back. My father took a microbiology class from a local college (for his own edification) and as an experiment he grew bacteria in Listerine. Every dentist I've ever been to said the over-the-counter stuff is useless. You're better off using 50/50 hydrogen peroxide/water mix for 45 seconds or getting a prescription mouthwash.
I keep my toothbrush in a plastic toothbrush container inside a zipped plastic bag. It probably holds the moisture in, but it keeps the toilet spray out.
Havasu, yikes. Is that black mold? OSB is a terrible substrate for flooring. Some flooring installers won't put flooring over it or won't warranty their work if they do. Might want to put some luan over it and a moisture barrier on top of that. That way, if there is a leak you can pull up the floorboards and dry it out instead of letting it damage the floor. I plan to put a moisture barrier under my vinyl plank in the laundry room.
To stay on topic, here is the drain under Mom's bathroom sink
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