Confusing Military Words

[nitpick]
'Colonel" comes from the Spanish for “Column”, and referred to the officer that was placed in charge for that formation (the formation itself being an Italian innovation). A classic Cecil column discussed this, as does an excellent history of military ranks available for the Naval Historical Center website.
[/nitpick]
Otherwise, you are completely correct, of course.

Except my Chambers Dictionary of Etymology, which is usually the end authority, says about colonel:

He then says that the first l in colonnello being changed to an r is due to “dissimilation of two identical neighboring sounds.”

He also offers that both pronunciations (colonel and coronel) were used until the early 19th century, when “'kernel” became the favored pronunciation, but it remained colonel in print.

So, my take is that it’s Italian from the Latin.

…and so Gaudere’s Law comes to haunt another Doper…

So in baseball, why do only the pitcher and catcher make up the battery?

My understanding – which I’d appreciate having more learned etymologists correct if necessary – is that battery (from the French for pummel) was eventually applied not just to a gun, but to a collection of cannon, operating as a unit.
From there, it became slightly abstracted and applied to many more groups of items, working together, such as a battery of tests.
Early electrical researchers connected a bunch of voltage-producing cells together to get the higher voltage they wanted. They called this group a battery, implying a group working together.
Later, popular usage started referring to any electrical storage device as a battery, even if it was a single cell (as most flashlight and other batteries are).

I haven’t heard the term much in baseball, but it probably comes from a combination of the middle concept – a group working together – with some overtones of the cannon usage – as pitchers throwing balls are kind of analagous to cannons shooting cannonballs. I kind of thought it referred to the bullpen (i.e. a collection of pitchers), but I haven’t spent nearly enough time spitting sunflower seeds to be any more definite.

I think Quercus has the essence of it precisely. I’ve heard battery used to describe a group of similar items on occasion.

Battery, to mean a pitcher/catcher combo, was well-used by the 1880’s.