If I were to, say, nuke the Celts' 1-each of those resources, would those nukes destroy the entire squares' worth of improvements and cut it off long enough to delay producton to allow me to catch up? I don't believe I've used a nuke before, so I have no idea of the effects.
A nuke-detonation destroys all terrain-improvements, including roads, in the 3x3 area centred on the target-tile. It also pollutes all land-tiles in that radius, and instantly degrades them by 1 step, per the 'Pollution-effects' settings in the editor (i.e. Jungle/Forest [–> Grass] -> Plains -> Desert).
So yes, nuking their (Spaceship-part) resources would cut their access to them, but that won't help you if they've already finished the tech and started building the part (since the resource(s) is only needed to begin
a build, not to complete it).
As you might expect, the other damage done by a Nuke is also pretty severe: IIRC, a Nuke's B-value [=99?] is applied as if it were an A-value, against the D-value of every
unit and building in the 3x3 radius: so all D=0 units will die instantly, and most/all of the D>0 units will die as well (MechInf or ModArm may
occasionally survive a first strike, but likely with only 1 HP remaining); and half the population/ buildings in a nuked town will be destroyed.
Wow ... that actually sounds really cool. Does merely nuking the capital destroy the Ship, or does the capital actually have to be taken?
To destroy the ship, you have to take the capital, otherwise 'all' your Nuke will do (in addition to the effects described above!) is cut it off from their trade-net.
While this would
be sufficient to stop Brennus from building the last ship-part in his capital, and might
cause disorder there (from the loss of Luxes/ Happy-buildings), (1) it won't stop Brennus from completing his last ship-part somewhere else (and launching his ship), and (2) only one road would need to be rebuilt to reconnect the capital anyway.
If you can't reach his capital in time, then to ensure that he could not research/ build his last Ship-part quicker than you can, you would have to nuke (and hence cut off the resource supply to) all
his core-cities, not just his capital.
It should be pointed out here, though, that Nukes themselves are immune to Nuke-blasts, so glassing his core-towns won't
destroy any Nukes that he has already built. So if you fire yours, you will then have to deal with the effects of all the Nukes which he will
throw at your towns in MAD-retaliation.
So is there really
no possibility of tech-trading to catch up with the Celts?
Or winning by the UN-vote? (Or are you still playing with that VC switched off?)