Here's my thoughts on the matter. Also, I'm a bit disappointed that the Relic was pushed back, as it was the part of the patch I was most looking forward to. By the time I joined the game, it was late in the 2.xx cycle, so once I reached end game I just didn't see the point in going through that grind, as I was still gearing up everything else with Soldiery and Poetics gear even by the time HW hit, and I still had plenty of content to get to that didn't require a top end weapon. Anyhow, my thoughts in regards to your points.....
Almost certainly required....
I find it doubtful that we'll need the ESO weapon to get the relic, and it most certainly won't require the upgrade. SE got to the point that they were making fun of all the rediculous requirements they set up in the past for Relics, and have been actively nerfing the previous ones at every chance they get. I don't see them putting the relic weapon behind an ESO gate that will take you two weeks of capping and then some to unlock. Not unless they remove the ESO cap, which would be a bit soon for that. Same goes for the idea of needing the upgraded one. On top of the previous ESO cap, you need the upgrade materials which currently only drop from Savage. I'm aware that they'll be putting in another method to get said materials, but even then, it seems a bit of a stretch to require you to get the strongest weapon currently in the game before you can start on your relic.....Eso item: it's because it's 1st job specific weapon we get. Not sure if we should upgrade to 210 one 1st.
It has been confirmed that those who have a completed version of the previous relic line will be able to trade in said weapon to skip the first stage of a new relic weapon. This trade in can be from any class to any other class, and works on a 1 to 1 ratio. For example, you could trade a PLD Relic in from the previous line to skip the first stage of a DRK Relic from 3.15. You can only do it once per Relic, but can do it for as many times as you have completed 2.xx Relics. HOWEVER, I believe I read somewhere that Yoshi P stated that if you don't already have a 2.xx Relic in progress, then it isn't worth starting a new one to skip the first stage of a 3.15 Relic.....Zeta Weapon: (Optional) there were many speculation about SE is going to implement something so those who have zeta weapon they could skip few grinding but nothing has been confirmed yet. I am guessing all the pre 3.0 dungeon grinding part will skipped at least.
I doubt it, simply because either they're incredibly difficult when DFing a group, (especial for those of us with latency issues on runs like Titan), or they're pathetically easy when going in as an unsynced pre-formed group. I remember joining a Garuda EX farm a couple of weeks ago, with 1 tank, 1 healer, and four DPS that were all either BRDs or MCH, (I was the lone MCH, as usual). Even with a couple of players missing, and the lowest DPS classes, we were getting in clears in about 2 min. Unless they required you to DF it, it would be pointless to have 2.xx cycle primals, at the very least.....HM/Ex primal fights: it never have been a pre-requisite for anything. SE being nice about not including any of EX primal fight for any of main story quest nor relic quest but since we built up some ex for a change? I really hope not as i won't be able to clear titan ex with my ping.
This I could actually see, at least as an attempt to revitalize the Crystal Tower trilogy to some extent. Right now LotA, ST, and WoD are as dead as can be.....24 man Raid: do do a CPR to dying 24 man raid, SE may want to include this as part of relic quest. void arc will probably not going to be included i think.
They absolutely would not require you to do something that couldn't be done solo or in the DF. There's too many players who either aren't in a FC, are in small FCs, or are in a FC that doesn't have airships unlocked. We MIGHT get some of the DF Island Exploration as one of the last stages for the relic, but again, that's content you can DF....Airship exploration: I am not sure if this will be required as part of relic requirement. I am not sure if SE would penalise anyone who have only few people in FC.
I could be wrong in all these regards, but only time will tell....