Some topics are only available after learning something special from another NPC, or through completing quest goals, etc. Cutscenes use full voice for all characters, yes. In-game conversations are text only. There's rain and snow in some levels, yes. It does not change in realtime, however.
Weather is used by the artists to set moods in the levels.
Weather and time of day varies according to where in the storyline the player is. Anything the player picks up during the game goes into this pool until dropped, sold, destroyed, etc. There's no limit to the number of items the party can carry this way. Available slots include Weapon, Shield, Torso, Pants, Boots, Gloves, Neck, and two Ring slots. All equippable slots change the character's appearance in the game view, except the Neck and Ring slots.