-Coughs- Warning: Big Post incoming.
One of the big reasons the UOC couldn't build adequate defenses in time was partly due to OOC disinterest and ICly Yamatai taking pretty much 'all' of their technology back. Bodies, ships, stations and so on. And if I remember correctly Yamatai hired Privateers to raid UOC territory (Or that may have been taken out, in what I refer to as 'The Dark Times' which is basically the drama wars of 07'-'08.) So it was a combination of the UOC's inability to adequately build defenses due to needing to build an entire interstellar infrastructure from almost nothing. Compound that with pirates, privateers, raiders (And remaining SMX forces that did not go away in the magical meta-plot gates of endgamedom.) swooping down on fledgling tradelanes and so on the ships that Yamatai took back could've been useful.
However, the UOC had built a few vessels by this time but couldn't patrol all of their space with a handful of untested prototypes.
With the arrival of the Lorath (And forgive me, but my memory is hazy around here due to wanting to block this whole sordid affair out.) we added a few million Lorath, billion or so Helashio, and their clones and so on to our already strained resource base.
There is going to be resentment for Yamatai, no ifs, ands or buts about it and no amount of admin or modfu can change it. You will find it somewhere. And I'm telling you this as a former player, and FM of the UOC. The 5th XF felt morally wronged by Yamatai at large while still under their rule due to the Battle of Taiie and other factors. In IC time it has been a little over four years since then.
Logically they'd still hold some sort of grudge and resentment especially amongst some of the former military. I'm not saying it is entirely Yamatai's fault, but they are not faultless here. The UOC is to blame in part as well, do I know that the UOC'd be still around and strong ICly if they'd not had their ships, weapons and such taken away? No idea, from an IC standpoint they'd likely still be around if we didn't muddle it up with the OOC business.
Can I say that Yamatai's demands for their stuff back led to the downfall of the UOC? In part, and only in part (Along with the series of events that followed which were not their fault). It was a domino effect, an already small, weak newly founded nation was left defenseless, mostly grounded and with a substandard economic base/military force at the time that could have been bolstered by those ships and weapons. That may stick in the back of their minds, in addition to the stuff during and after Taiie.
Yamatai did not make the NMX, but they had a hand in making its predecessor, the SMX, which in part led to the development of its successor the NMX, some, not all may feel that "Hey, we should be mad about this!" but likely a small part only (And if they actually KNEW). The majority may still feel wronged, and hold some not a lot, but some resentment due to other things along with the fact their nation just got wiped out.
Tempers will be flaring, but that only comes after the shock, guild, and depression has had time to fester and something else will come up. Given time they will more than likely calm down and come to their senses, blame maybe given during the start, but near the end the UOC survivors will want only one of two things. A new home in which to pickup the pieces, and possibly to avenge the loss of any loved ones when their nation fell. As is normal for any person. I'm not going to hand out ratios on which for fake people, and we as real people should take these things into account if we were put into their shoes. We'd be mad, angry, and looking to the nearest source of resentment that is etched into the back of our minds, blame them, hate them, but then come to realize it is fruitless, futile, and then move on. Or, mope, feel sad and so on. Or indifferent, it varies too much to put into words.
As for the Nepleslian views, that is free to interpretation from player to player. Most Nepleslian players I have dealt with have had their characters dislike, or partly distrust/loathe Yamatai. So unless there is an OOC campaign to change this, or some IC Pamphlet campaign or whatnot to 'change' these views, it is going to vary, and vary widely. Don't quote me on the latter of my huge post. It is just what I have observed over the years from Nep players in IC. But with the war, I can't paint a clear picture.