One thing I have always found curious about this conflict is its timing and the fact that Spain had no allies, such as for example Mexico or even Germany, where one of the ego trips of Kaiser Wilhelm II was to seize Panama (I am aware that that was post this war, but you get the idea).
Spanish foreign policy on that era was called "Recogimiento" or "seclusion", a kind of isolationism. The borbonic restauration regime (1875-1931) was centered, most over all, on maintaining spanish political stability after decades of uneasiness, and tried to avoid foreign compromises in order not to break internal social peace.
Of course that had the consequence of complete loneliness when spanish american war broke out, and this made looking for new alliances a priority on the aftermath of the disaster.
Regarding the USS Vesuvius and her experimental artillery, the same kind of pneumatic dinamite gun -Hmmm maybe smaller in caliber?- was fitted on the Holland
to fire grenades underwater. Both J.P. Holland and Zalinsky wanted to try the new weapon bombarding Spanish coastal batteries at Havana with the submarine submerged, at night, and thus completely invulnerable. USN refused to do so -wisely, the success of the strike being doubtful at best, didn't worth the risk to lose the boat.
Those fixed pneumatic guns had no arc of fire and were useless at naval combat; When shooting at static targets was always short and inaccurate as well.
Incorrect: The USS Kearsarge & Kentucky, which introduced the 2 story turret, had been authorized back in 1895 & their keels laid in 1896 - well before the Span-Am War. The first generation US BB's, the Indiana class & the Iowa 9completed in 1895 & 96) all carried an intermediate armament of 8 - 8" guns in 4 turrets at the corners of the superstructure. This arrangement however, left little room for secondary guns, 4 - 6" BL on the Indiana class & 6 - 4" QF on the Iowa. The 2 story turret was an attempt to preserve the 4 - 8" intermediate guns on the broadside while providing more room for an effective secondary armament,in the case of the Kearsarge class 14 - 5" QF.
According to Friedman, the unusual (By the time they were designed) heavy secondary artillery layout of Indiana class battleships was due to the lack of appropriate medium caliber quick firing guns at the moment in USN; The designers exchanged rate of fire for striking power in that case. USS Iowa followed the same path and USS Kearsarge definitely introduced a proper QF secondary battery in the form of 14 - 5 inchers; Retaining 8 inchers was a wise move as it proved a very effective caliber at span-am war, and predreadnoughts would soon start having intermediate artillery between QF guns and heavy guns.