If by "Underworld" you mean "War of Underworld", then you're talking about the SECOND half of Alicization.
There's three seasons in total. SAO, SAOII, and SAO Alicization. However, SAO Alicization is so big that it had two mid-season breaks, so anime sites have arbitrarily divided it into three entries, despite it all being the same season.
The anime skipped nearly all references to Shinkawa. You'll have to read the novel if you want to read about her visit. And it wasn't the hospital.
Regarding your first question: Sword Art Online Alicization is a 4 cour (4 x 10-14 episode bundles) 3rd season for SAO that continues the story of the second season. Since the 3rd season is huge, it's divided into 3 parts by seasonal breaks to give the animators time to produce a quality product (the animation for later episodes is still ongoing when an anime season begins airing, so the anime airing eventually catches up to the animators) and the season consists of only one arc, but it's so big that it's divided into two sub arcs for manageability and thus gets a subtitle midway through the season, but it's still just one hugeass season. The final eleven episodes of the season will begin airing weekly starting from April 25.
TL;DR: Yes, it's just a big season 3 of SAO.
Regarding your second question: it's available on Crunchy, Funimation and Hulu. Netflix may not have all the episodes yet, since they prefer adding episodes in huge bundles.
If you're watching the English dub, the third cour is currently still having its first airing on TV, so not all of the episodes will be available.
Kawahara wrote it, but posted it on a special page where he hosted various fanfics from his readers. It was never posted alongside the web novel, let alone the light novel.
He wakes up at the end of Alicization.
For the record, it is possible to copy Fluctlights. However: 1) you can't copy Fluctlights of dead people. If they're dead, there's nothing left to copy. The STL isn't some magic tool that can just bring back the dead. 2) Copies can't stand finding out that they are copies. It literally breaks them.
I'd hate to break it to you... but there won't be any new season next year. What we're getting is a continuation of the current season after a break. We're still on season 3, not season 5.
The arc after Alicization is Unital Ring, not Progressive
Progressive takes place during the Aincrad arc
Whether or not Progressive even will get an anime is an open question right now.
Unital Ring is the arc AFTER Alicization...
Moon Cradle is an Alicization side story that isn't relevant to the main story. It basically just a story that takes place during a time skip in the Alicization arc.
They are not married even in the anime...
Also, for reference, the first volume of the new arc has been summarised in detail already. https://dreadfuldecoding.blogspot.com/2019/02/sao-v21-summary.html
Near the very end of the Alicization anime. Asuna's coming alone isn't enough to wake up a guy who's basically killed his will out of self-guilt.
See beginning of Alicization... Shinkawa wasn't the one to get the job done, but their partner did...
Gabriel was hired by the NSA to retrieve Alice as the most advanced AI produced by Rath. However, his actual goal is to retrieve Alice for himself and to create a personal virtual world that he plans to populate with "high-grade souls" like Alice's and those of SAO Survivors.
> While i can't claim to read the SAO manga (like at all)
SAO is a light NOVEL, not manga.
>But once Aincrad was revived (thanks largely to the World Seed) they all always seen with their marriage bands/weddings rings on.
Regular stat boosting rings. ALO doesn't even have a marriage system.
> but given that at the end of the alicization arc that kirito was still comatose
The Alicization arc doesn't end with the Quinella battle... Alicization War of Underworld is STILL Alicization, just that it now has a subtitle.