Jewel clvl req is only based solely on affixes, right? Being that there's no base req clvl (i.e. norm, except, elite)
So <=20ed will always be <=clvl 30?

yes

no rlvl for the base, rlvl depends only on modifiers. rlvl might also be changed if the item you socket it in was upgraded

required level = 0.75 alvl

https://classic.battle.net/diablo2exp/items/magic/jewels.shtml

"Rusty" = alvl 13
13 * 0.75 = 9.75
int(9.75) = 9

therefore required level = 9 for 20% ed jewel

no, rlvl is hardcoded. the formula you quoted is correct for many mods, but not for all

for magic / rares alvl determines the required level if greater than required level for the base.

for uniques or crafted it's different

no. not the alvl determines rlvl, but the affix itself does (in the form of a long hard-coded list). there is no general interconversion formula between alvl and rlvl. arreasummit listing the factor of 0.75 is a huge mess-up by blizzard, posting incorrect information about their own game

what's an example where the integer value of 0.75 alvl doesn't equate to the rlvl for a rare or magic jewel?

many instances of ml for example: circlet/amulet with high ml, rings with high ml (there are actually affixes with same alvl for rings/amulets, but different rlvl), melee weapons with ml and so on. see here:
https://www.theamazonbasin.com/wiki/index.php/Mana_Stolen
iirc there is some discrepancy for life as well, but i am not sure. havent generated a whole list yet, otherwise one could just use a search function

what a buggy ass game