Some things I don't know, some I think simply aren't known, but I'll try anyway.
Norn Queen but how big in comparison to vehicles/ imperial ships + buildings.
Size isn't mentionned as far as I know, except by saying that they inhabit massive chambers inside the Hive Ships. They don't
have to be enormous, since most/all organism they birth are Ripper Size, but I assume they would still be massive, otherwise they're no need to have them in 'massive chambers'.
Would it be possible to cut one off from the Hive mind if sufficiently weakened?
Since there is psychic dampeners used by the Imperium, yes, certainly.
if a warp storm surrounded a fleet, would it overpower the connection to the hivemind?
Dunno. I guess that would depend on the author.
If that happened, would the Norn Queen retain its 'intelligence' or would it become feral?
Being cut from the Hive Mind would make her go feral, but that doesn't mean she'd become like Instinctive Behaviour gaunts. The big bugs are very intelligent even on their own.
Genestealers How likely is it that a planet with, perhaps, a mechanicus base, large-ish military protection and elite guards/ an industrial hive world could fight off a Genestealer infestation without protection/ reinforcements.
Very likely. Genestealers are the scouts, spies and infiltration specialists of the Nids. Their function is to try and infiltrate a world and reproduce until they spawn a Broodlord, and thus demonstrating to the nearby Hive Fleet that the world is rich enough to support advanced lifeforms that are worth munching on. They attack to disrupt the planet's inhabitant right before the infestation starts. If they are discovered before the Hive Fleet is ready to strike, or before the broodlord is spawned, then most likely, the planet's inhabitant will win the fight.
if a Genestealer cult could take hold, how long would it go unnoticed?
Well, until they either get caught or until the Hive Fleet invade. Genestealers and their Cults dont wage war on their own unless something went wrong or unless it's part of the Hive Fleet's strategy.
would it be able to enslave an entire population?
It would have to be a very small population.
General Could the Tyranids produce a organism that can latch onto humans/ augmented humans and access their brains to use their knowledge/ use their physical bodies?
The Lictor already does the whole brain-slurping-to-gain-knowledge thing. I imagine that it would be easy for the Hive Mind to design something to do the other. But psychic Nids (especially the Broodlord) are already capable of inducing 'xenophilia' in human beings (and probably other advanced xenos). Scientists who examine Nids specimen often becomes obsessed with how perfect the Nids organisms are, and then start doing very stupid things like releasing those specimens...
Can normal Tyranids be born without a hive fleet ? with or without a Norn Queen?
Depends on which source you base yourself on. Lexicanum and older fluff states that only the Norn Queens can breed Nids larvae. Newer fluff (and the
IA books) indicates that even Gaunts could alter themselves in order to start breeding, when they are cut off from their Hive Fleet.
I think you would find The Anphelion Project
IA book very interesting. Its about an inquisitor's mission to a Mechanicum biological research world where they have a sort of Jurassic Park for Nids. It shows very well how, even without any space support, and starting with only small organisms, a Nid infestation can overrun a world in a matter of weeks.