A122: No. Gaseous form explicitly states what conditions/attacks you become immune to. Namely, poison, sneak attacks, and critical hits. You would still lose your turn if you became stunned, although you wouldn't need to drop what you're holding on account that you aren't carrying anything....

A123 No. A Ring of Invisibility allows to wearer to cast invisibility on themselves. According to the PFSRD, invisibility ends when a creature attacks, not after they've finished their action. The first attack of the pounce would gain the benefits of sneak attack since the target would be denied their dexterity bonus to AC due to being unaware of the attack. From then on, the target is aware of their attacker, (assuming this is not a surprise round, but then you wouldn't be able to perform a full-attack action anyways), and you are no longer invisible.