Hi Mikebenedict, thought I could hep you with a few answers. As for the totality of numbers featured as the Number of the Day... not sure what's the reason behind featuring only said numbers for so long, but I know that they finally decided to feature the numbers 0 and 13-20 after María's and Luis's wedding. I remember the honeymoon episode where they're driving in Oscar's Sloppy Jalopy and arrive at this camp area where they're bothered by Sonny Friendly. That episode was sponsored by the letters S and X and the number 17. The highest number they've featured is the number 21, with a poem by Ernie as a sketch and the story of Numberella as segments for that number.
As for sketches featuring the number 1, there's no Number Painter, Pinball Count nor Circus Ringmaster sketch/segment for this number. There have been various bits and segments since then though, like the western song sung by Johnny Cash with Big Bird, the Count and Countess Dahling von Dahling; the episode where a number 1 didn't want to be the number 1, the gag with Patrick Stewart where he gives the number 1 his classic ST TNG command; and an arcane song I remember that has to be in the Sesame vaults.
This sketch had the female number Muppets for the numbers 10 to 1 with lightbulbs on their outlines, all lined up for a sort of pageant. The number 1 was extremely shy and didn't want to perform at first, but was then coaxed into singing and gave it her all.
As for the Number Painter's 9 sketch, can't say for sure. But it's not rully that far-fetched, if you've been to NYC then you know that there's a street with a store (maybe a bookstore, not sure anymore) that has a big red number 9 stationed on the sidewalk in front of it.
Hope this helps and have a good weekend.