Regarding the F5/F6/F7/F8/F17/F18/F19/F20 block of 8 keys (which are Pebble in the standard key sets). The wording on the “New Custom Legend Keycaps and Extra Keys” page makes it sound like all 8 keys can be ordered separately for $24 (at $3 apiece). However it also makes it sound like all 8 keys can be ordered in only 3 colors (Pebble/Industrial SSK Blue/60% Dark Gray), with Pearl not being available. Can you confirm that this is correct or incorrect?
Sorry the page was not clear. Those keys can only be ordered as part of the complete set, and only in the default colors for that set (pebble/blue/dark gray). Mix and match or individual ordering is not possible.
So to confirm, all 8 of the keys (F5/F6/F7/F8/F17/F18/F19/F20) cannot be ordered separately? Or just 4 of the keys (F17/F18/F19/F20) cannot be ordered separately, and require the entire F13-F24 set to be ordered in one of 3 color options (Pearl+Pebble/Industrial SSK Blue/60% Dark Gray)? Which would make ordering 16 keys for $48 possible (F5/F6/F7/F8 + F13-F24)?

No F keys period? I can see you have just added F13-F24 to the keys that cannot be ordered individually on the page, but it only says that keys from “front printed F1-F12 sets” cannot be ordered individually. There is no standard F1-F12 set available to be ordered. If that is truly the case, then I am honestly quite shocked that this question has never come up before. I would recommend rewording it further to specify exactly which standard keys can and cannot be ordered individually. You could even highlight the keys on your standard key set picture if that made things easier ( https://www.modelfkeyboards.com/wp-content/uploads/2019/02/F-XT-Base-Set-render.png ).