The "fog" was a government attack.

I doubt I'm the first to guess this, but I didn't see anybody make a post about it. I believe that the "fog" over Esperia was a chemical weapon deployed by the government.

When the Holy Knights were taking over Elbaf, they said that they wanted to Destroy Elbaf's Culture, which is why they tried to burn the Library.
Esperia's culture is, obviously, Music... and the Fog very specifically rotted their instruments. Seems much too targeted for a random weather phenomenon if you ask me.

I have seen a lot of people theorise that Candelle isn't really dead, and may have been taken by the government. I think it's totally possible that the fog also served as a way to fake her death.

I know Mom Deaths are dime a dozen in one piece, but they're usually more weighty and dramatic than "sudden random death by Fog..." Except for, like, Usopp's mom... and uh... oh, you can probably find countless random deaths of moms, but I maintain that MOST of them involve either the mother being tragically and dramatically killed, noblely sacrificing herself for her children, or both.
(For example, Sanji's mom gets sick after taking pills to stop his genetic modifications, Ginny dies due to sailing during the day to get Bonney to Kuma, Rouge dies in childbirth after delaying Ace's birth long enough to save him from the navy.)

I also think the inability to pay Heavenly Tribiute was calculated. Either the government specifically wanted Slaves from Esperia, in the same way they wanted the warriors of Elbaf to be slaves - Or, they knew that Reuven wouldn't agree, and they wanted a legitimate excuse to destroy a Government Nation (in the same way they needed proof of Poneglyf research before they could destroy Ohara.)

I think the most important thing is that The Celestial Dragon Was Not Incidental. No WAY Brook just casually mentions they had a shipwrecked CD and that never comes up. I theorise that this was the reason behind everything that happened after: either the Celestial Dragon's own petty desires caused this, or perhaps, like how Garling discovered God Valley, this CD discovered some secret bloodline on Esperia that warranted it being wiped out.
(I assume it'd be Candelle's bloodline, but people have theorised that Brook or Reuven are davy clan so... go off.)

The celestial dragon was probably either somebody from the Manmeyer family, or perhaps Garling's Father (in which case, I further theorise that Candelle was the mother.) I think it's highly likely that they're a Holy Knight.

And I think the mysterious smoker could be that very Celestial Dragon, but he also might be the Admiral that Rocks ended up Killing.

And that is ALL! Please tell me what you think!