1. If you have those gears for new character at early game, it's good. -X res work for all dmg type res. Fragile alone only reduced enemies physical dmg. RR effect on items general works on weapon attack hit, so you need using weapon to hit enemies first so can apply RR effect on them; RR effect on non weapon( armors, accessories, artifact, skills) can works with skill like Throwing Knives and Blade Barrier of Rogue too.
2. It just worth to farm it if you want collected. That set's just Epic, so it not useful much in late game.
3. If you have it already, just use it. If not, continue play to lvl up your characters, you will find it somewhere with some characters.
4. You want make a Summoner build focus to Physical dmg => you should prioritze on +%physical dmg, -%recharge first, +skills/all skills then casting speed ( no need to max, just enough to cast it fast), then other defense method ( because if you have good -%recharge, +skills you can use Briar Ward often, and its a great defense way already).
+ flat Fire, %ADctH dmg only works for weapons attack hit. +%fire dmg can be optional because Eruption and Volcanic Orb deal both Physical and Fire dmg.
The most important res is the res that enemies you fights at current moment, feel free to change gears for specific enemies if you don't have gears with good res for all.
5. Yes, RoF can be good at early game and at its low lvl; late game and and high lvl of RoF noth worth by its high energy cost.
Eruption, Volcanic Orb (Fragmentation), Brimstone ( of Earth Enchanment), Volativity, Soften Metal (of RoF), Plague skills tree ( 2sub skills is more important than Plague itself), Refresh, Meteor Rain are the most important skills help you deal dmg to enemies. Strength of the Pack is good, but if you play xmod, sometime your pets die all, so it good but need support by other skills and gears, tactics if you want keep SotP works all time.
6. Fire Nova deal Fire and Burn dmg, can be good at procs any grants skill activated on attack on items, gears, ... so it not the most priorities if you don't fulfill those skill above first ( because you focus on Physical dmg, right!).
7. Yes, you can do simple as you mention. Or more steps like VO => Plague => Briar Ward => Eruption => Meteor Rain => Refresh => repeat,...
8. Yes, Heatshield grant you Fire dmg absorption and 15% physical dmg res.
9. Subsceptibiliy is RR by absolute. Its can stack with other RR by flat or by %. It good and important.