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You know, just because Ending C has Yggdra and co. starting Ragnarok doesn't mean that it's THE Ragnarok we see in Riviera. My personal theory is that YU would end in the "canonical" A ending, but that Ragnarok would eventually be started by demons some time afterwards (from hundreds to thousands of years, we don't know). Marietta, the young angel we see in YU, would be kicked out of Asgard at around this time.
Asgard, Utgard, and Heaven's Gate are all floating islands--Ancardia was originally a floating island but was expelled to Midgard (the human world) as we notice.
Midgard is where the humans live; my guess is that after Ragnarok, the Sprites (such as the Undines we see in YU) got moved to Utgard/Riviera to avoid conflict with humans. Remember, the Sprites were enslaved by the demons and fought for them in Ragnarok.
You can see this stuff a lot more coherently in The Tainted, where the story of YU gets referenced a few times by Celina, but yeah. Sorry if that made no sense.
Basically I think YU and Riviera take place in the same universe, but different places and different eras. 'Cause there are just WAY too many Riviera references in YU, from Banangos and "kokori no mi" (translated in YU as "Kokorinut" while it's "Applecot" in Riviera) to Mjollnir (which they say is a Diviner) to Nessiah's design (you just know that it ain't coincidental that he looks so much like the Magi...) and Marietta's cameo at the end, for them to be just references.
And since we don't know the life span of angels, the Marietta we see in YU could easily be the same archangel in Riviera or some relation of hers.