What do each of the element & status symbols mean?
When I use Libra, I can see icons depicting those elements against which the enemy is weak. But some of the symbols are so abstract that I can't figure out their meaning: A "Skull Defender" is weak against a white circle:
But what does "white circle" mean? Is there a guide somewhere that I'm not seeing?
That looks like the https://finalfantasy.fandom.com/wiki/Holy_(element)#Final_Fantasy_XII element to me.
Regarding the other part of your question, the following is a list of status effects in FFXII:
Note: For Doom and Petrify, a countdown (4.5 seconds per tick) starting from 10 will appear by the health bar above the unit. The character will be KO'd/turned to stone when the count reaches 0.
For more information about status effects, see https://finalfantasy.fandom.com/wiki/Final_Fantasy_XII_statuses .