1) not exactly intentionally bu t if you pick the right trusts it can be done
2) No
3) Up to a limit each week you can sell gear bought with Sparks of Eminence and Prize Powder bought with Unity Accolades for gil at NPC vendors, after that its mostly crafting or mercing
4) I dont play SAM can't help you
5) All story content can be solo'd with trusts. The end parts of Rhapsodies of Vanadiel you may want some help
6) Yes Yes and Yes. Some items in non-visible gear slots do still have meaning but for visible gear slots only item-level 119 gear is of use when you reach level 99. You can use the Style Lock feature though, to keep the look of old gear you like. Having item level gear will also make your trusts stronger, so get 119 stuff in all visible gear slots asap