(Spoilers up to ShB) So, what exactly DID our Urianger DO?

[NRieh]

The stuff he regularly guilt-trips over isn't Minfilia ending up bound to Hydalyn to begin with, but his actions in manipulating Elidibus, the Warriors of Darkness and the Scions to bring their and our crystals together to restore some of Hydaelyn's power* and send Minfilia to the First - setting her up for that mission without giving her a say in it, even if she was probably willing to do it anyway.

He definitely seems to overstate what a terrible thing it was for him to do. Like apologising profusely for the current lesss-than-perfect state of the First when there wouldn't even be a First right now if he hadn't set up Minfilia to intervene.

The thing is, the entire plan of saving the 1st literally hinges on the very fact of Minfilia's sacrifice at the end of ARR, and it comes (as I mentioned in OP) from lots of random factors. Apparently, no Minfilia caught in AOE = no Emissary, no averted calamity, etc. I'm trying to figure out the cause-effect, and whether it was as random as it seems. We can always assume it's just the storytelling convenience, but I'd rather believe that some inner logic and explanation still exists here.

Could it be that Urianger was, in fact, somehow aware of Minfilia's adventure long before WoL's Antitower tour? Long enough to figure out her place in this plan of his, maybe? Like, "Hey, look. We've got the right person for that Emissary job, and we could boost our Mother Crystal with powers? And we could annoy some Ascians, while we're at it? Let's make it happen! F*, Elibidius. Sorry, Ardbert."

* Hydaelyn seemed on the verge of failing altogether after the Antitower, so weakened that Minfilia was unable to hold her physical form after speaking to us. The rush of energy from the crystals allowed Minfilia to separate from Hydaelyn again and become an emissary rather than simply the Word of the Mother.

True. And if I had to make an assumption, I'd say that it was Hydaelyn herself, who had guided Urianger and Minfilia both. First, she seized an opportunity to reach out and summon Minfilia's resources to her side. So that she could later reach out to Urianger ( either directly or via her Word of Mother), while WoL was busy with knights&dragons.

I've looked up the lore description of that WoD quest. the Archon confesses to having orchestrated events to grant Minfilia her freedom, that she might then use her newfound powers to prevent the First from being lost to the void. Though this revelation prompts outrage in your allies, she reassures them that she is happy to have been given such an opportunity. Not much of the specifics, I guess, but better than nothing..

[Iscah]

The thing is, the entire plan of saving the 1st literally hinges on the very fact of Minfilia's sacrifice at the end of ARR, and it comes (as I mentioned in OP) from lots of random factors. Apparently, no Minfilia caught in AOE = no Emissary, no averted calamity, etc. I'm trying to figure out the cause-effect, and whether it was as random as it seems. We can always assume it's just the storytelling convenience, but I'd rather believe that some inner logic and ewxplanation still exists here.

Could it be that Urianger was, in fact, somehow aware of Minfilia's adventure long before WoL's Antitower tour? Long enough to figure out her place in this plan of his, maybe? Like, "Hey, look. We've got the right person for that Emissary job, and we could boost our Mother Crystal with powers? And we could annoy some Ascians, while we're at it? Let's make it happen! F*, Elibidius. Sorry, Ardbert."

You're looking too far back. Why do you think Urianger had anything to do with Minfilia ending up in the aetherial sea? He didn't, and he had no idea what happened to her until we started tracing the lost Scions' aetherial trails in Heavensward. It's only once we work out what happened to Minfilia that he can start thinking about what to do about it – and I don't think we had that worked out until 3.2 just before we went to the Antitower and came back to relay what we saw there to the Scions. That's the point where Urianger would know that Hydaelyn is fading and also understand the exact nature of Minfilia's current state, at which point he starts manipulating events with that specific plan in mind.

He was in contact with Elidibus already, of course, but it may have just been a more general "play along and see how much information I can extract" venture at that point.

I'm not following the scenario you're picturing in your second paragraph at all.