The chances are that when the weather becomes colder, Nura will remember onw hich side her bread is buttered, and will return to the warmth and comfort of her own home, but fully understand your distress and concerns.
As I say, this time last year, we lost one of our cats to a road accident. We took on another - funnily enough, he is a mini maine coon (not pedigree but clearly his ancestry has outed itself in full, save for the fact he's quite small lol!) and our Tinks did very much the same as your Nura.
We set up the spare bedroom as Moray's den, and it was his "time out" zone. We'd pop him in there in the evening after he'd been fed and played with in vigorous games of Flying Frenzy and chase, and then Tinks had his quiet time, so when he came in, he would find the house peaceful, and he got lots of fuss and attention. OH and I would take it in turns to spend time with Tinks and time with the kitten, which even extended to who spent the night with him in the spare room. When Tinks was out, Moray came out and got his time in the rest of the home. It wasnt too long before Tinks could be found lurking outside the spare bedroom door, sniffing and watching the shadows under the door.
His curiousity eventually got the better of him, and he wanted to see the kitten inside the room. he still didnt like him much, but he wouldnt immediately run away. Moray was, thankfully, very patient, and would extend an inviotation to play to his big brother, who - more oftent han not - would run off and watch from a distance. Then again, curiousity got the better of him, and after a while, he wanted to join in the fun, and would have a ilttle play - half-hearted at first, but then more daring. It took around three weeks before they ended up at ease, and it was a testing three weeks - many on here can tell you how many times I ended up either in tears or in a battle with my OH, who felt initially the better course of action was to return Moray to the shelter.
I'm very glad we didn't, and the patience paid off in the end.
I hope that Nura and Zenobia will also settle given time, and using some of the hints and tips you can garner here and there.
We also plugged in two Pet remedy diffusers (or feliway if you can't get them) - one upstairs and one downstairs, and I certainly think they helped.