Depending on how much you play, you can go “fishing” for the green pieces of gear by ascending Uncommon gear to Rare. With the 1.0 changes to Eveline, she can rather easily craft random Rare relics and then convert them to the one you need for the gear piece you are pursuing. It’s a lot easier than trying to chase down a specific green Rare piece of gear from a dungeon, although you’ll need the minor and lessor shards to raise an Uncommon gear piece to 15 for each dice roll attempt at trying to roll green for the elemental attribute.
For now, it’s both to slow players down and to avoid them having “perfect” pieces of gear this early in the game’s life. Somewhere down the road there will be powercreep to address this, but that’s probably a long way away.
Depends on the playstyle you want. Weapon effects tend to dictate the playstyle of the player and which gear will match up with the weapon best.
For example, your Royal Longsword is a build-up weapon for Power on green gems. Not a bad choice. If you go with this, why not pair it with a Warlord’s Belt so that you also gain Power on skull gems as well?
Some weapons channel Mastery into Power (and also the other way around) for big melee or spell damage hits.
Other weapons, like the Essgard Flail, generate skulls for extra damage when matching specific colors, like green.
Up to you to decide which weapon style agrees with you the most. 