Why? The nuclear arsenal going off would be totally insignificant compared to the explosion of two full sized planets colliding with such force that they are both destroyed. It is pointless to add it in there. It's just a dumb plan - the sort of thing a child thinks of.
Two planets colliding would produce kinetic energy and heat energy. Galactus isn't shown to absorb kinetic energy. I suppose he could absorb heat but I doubt he gets much from it.
The nukes would just add some radiation for no significant increase to the explosive force or heat of the planetary detonation. Galactus can definitely absorb that, and while it's not living energy from planets he might still get some basic juice from it.
no it wouldn't as we have no idea as too how many nukes Thanos used. Sizeable could mean anything 10's to 100's or even 1000's. Plus you have to take into account the happened against a drained Galactus.
Unless it's stated the writers don't give a shit about what energy is created/released from such a explosion. The whole point was the lvl/impact of the blast and Galactus doesn't just go around absorbing blast energy as shown in his countless battles.
Lesser bombs Pit could carry in his hands dropped on Hunger (speaking of, so am I) were said to take out half a planet iirc. Which you'd think a sizable nuclear device would do an untold amount more.
There's also the part where Galactus tanked a supernova in the same arc...
Galactus's level is irrelevant because the nukes don't add anything significant to the explosion in the first place.
Even if it was thousands or 10,000 or 100,000 nukes it wouldn't add anything significant to the force of two full-sized planets smashing each other to pieces in a collision. Nukes aren't just aren't that powerful when compared to an explosion on that scale. Even if they had been adding antimatter which is far more efficient it would still have to be a pretty large amount of the stuff.
Starlin just thought it sounded good, I doubt he had a clue about it. But like I said... it's comics, so whatever.