Pathfinder 2E Combat Maneuvers – Does ‘Improved Knockdown’ Trigger ‘The Harder They Fall’?

First, to the title question: No, uncanny dodge does not make a character completely immune to ranged sneak attacks. There are ways to make a creature flat-footed aside from catching them before they act, and ways to deny Dexterity to AC without relying on invisibility.

As for uncanny dodge vs. surprise attack, I agree with your second interpretation: uncanny dodge prevents the character from being flat-footed. It does nothing to prevent the character from being treated as flat-footed by a particular assailant. But it’s definitely ambiguous enough that I wouldn’t rely on this interpretation without discussing it with the DM (or explaining it to players, if I were DM and one of them thought uncanny dodge protected them from surprise attack).

No

All weapons have weapon category (pg.279) such as simple, martial, or advanced. Weapons also have other traits, such as agile. The Rogue sneak attack feature (pg.181) requires the use of either:

• A melee weapon with the agile or finesse trait
• An unarmed attack with agile or finesse trait
• A ranged weapon

As you said, the Ruffian racket also allows you to sneak attack with any simple weapon.

A weapon you utilize through the Unconventional Weaponry feat can only be used with sneak attack if it fits into one of those categories. Unconventional doesn't change the weapon category, it only changes how you calculate your proficiency.

So no - it's not true that any weapon accessed through unconventional weaponry qualifies for sneak attack, because it's not automatically a simple weapon. However, if you choose an uncommon simple weapon (such as a clan dagger or katar) you can use it for sneak attacking.