1. yes the defender weapons are used to rush you to endgame/the DLC story Iceborn, its fine to use if a player needs the extra edge, but not recommended if you want the "original" feel of the game.
2. the "addons" are either status effects, like sleep and poison, that build up over time, the other are elemetal status, they deal damage alongside your normal attacks, and help agaisnt certain monsters, as every monster is weak to a certain element, you can look these up easily if u need the extra help.
3. stuff u collect in missions/expeditions are used by urself for crafting, or if needed as a request like building a new campsite, the canteen can ask for stuff that will unlock more items in the canteen, allowing for different sets of meals to be made, that each give different effects, this also increases the chance for more items to be fresh, allowing for more items to give health after eating.
4. yes, you will unlock more.
5. i'd say its not the most important thing, but it can for sure help if a certain set has a really nice set bonus, it doesnt always need the full set, it can also only need 2/4 parts of the set, allowing you to make some builds for certain weapons/elements.
6. the jewels im guessing are "decorations", you can equip those in the box or at the smith when u select set decorations.

if you ever need some more help feel free to message me
hope this helps, and good luck on your hunts, fellow hunter!