- Since we never see anything but the one small corner of the world the show is set in, this is entirely plausible. The only hint we have that there's no-one out there is the fact Jack can't pick up any transmissions from his shortwave radio, but all that means is that no-one's capable of transmitting (or Jack simply has a run of bad luck and misses any possible transmissions).
- In the book, The Tribe: A New Dawn, there is portrayed a "United Nations Survival Fleet", a fleet of various ships that formed a floating city of a sort, connected through various bridges where at least somewhat recently adults used to live and try to find a cure for the Virus. Of course, they are all engimatically gone by the time of the book takes place. This still takes place relatively near where the Tribe left at the end of the series, so similar survival communities might be around the world.
- Trudy is Jane's daughter, as the Identical Granddaughter is in effect, skipping a generation each time. Her personality is similar to Rita and Loretta as well.
Basically it's one with a surviving British Empire which later became a republic and in which America never became independent. So they had a President, British money, American-twinged accents due to migration within the former Empire, and somehow set up the new capital in New Zealand as its far enough away not to be easily invaded in time of war. It's colloquially called "The City" simply because it's so important no-one needs to refer to it by its actual name.
- It's been a while since I saw this series, but as far as I remember you are correct. It was at some point elaborated on that their sonic guns for example were from a military lab - and my memory might be failing me on this one, but also I think irreplaceable; Repairable to a point but they can't manufacture more. Ram was the son of a scientist working in said lab.
- Ram broke his legs while jumping off a wall wearing a VR headset that was so lifelike he forgot it wasn't real. That was before the virus. So in the vague futurish world they live in, the technology was more advanced than the 90s. Though it was still commented on that the technos looked like they were more advanced than what was commonly around before the virus. But if we accept that excellent VR existed before, then the only thing really strange are the zappers.
You have a militant group of religious extremists (the Zootists) and Mega and the Technos seeiking to exert more and more control measures (via technology) with the excuse of counteracting the threat they pose. Does This Remind You of Anything?
The fact that Mega even goes so far as to force everyone to wear barcodes on their wrists is reminiscent of earlier theories which claim that barcodes were a precursor to the "Mark of the Beast". (Christian end-times prognostications often tend to overlap with conspiracy theories online...)