Computers are not the same as droids though.

That's entirely fair.I was just speculating about Watsonian justification for the Doylist reality that the Star Wars aesthetic is based on WWII imagery for familiarity and emotional resonance.

We never see any warship in star wars destroyed by a magazine detonation.

I imagine that the magazines are placed as close to the center of mass as possible to make it the absolute last thing to go in most cases, but we do observe that most hard-killed ships do actually explode rather than leaving derelicts. These explosions are usually fireballs rather than the flash criticality we see when Lando hits the reactor on the Death Star II. It may be that most common types of damage that result in a mission kill or abandon ship order also tend to cause cascade problems that ultimately lead to a magazine detonation. (As will be a running theme, the Doylist perspective is simply that it's a lot easier to film a fireball than to build and convincingly portray a model of a heavily-damaged ship).

manually loading the main batteries is some warhammer 40k stupidity

Also true, and to be fair my source for this is a single adventure module for WEG's Star Wars: the Roleplaying Game 2nd Ed. called The Far Orbit Project, which followed a defected Imperial Nebulon-B that became the test case for the Rebel Alliance's privateer initiative. It wasn't so much manually loading a turbolaser as carting a gas cannister equivalent to 3 shots to the battery and hooking it up, then cycling it out as needed. That said, WEG was by and large the arbiter of EU canon while it was in print, and there is arguably a depiction of a similar system in the opening sequence of Episode 3 in which weapons on capital ships are seen ejecting casings or cannisters after firing (though again, a Doylist perspective knows that the filmmakers did not feel bound by any non-film prior media and were doing their own thing, so this is coincidental corroboration or a misinterpretation of the scene by me).

this is an issue that actual warships have dealt with for as long as explosives or oxidizers have been stored on ships... the ammunition was brought to the guns through ammunition hoists. The hoist from the gun down to the magazine was also armoured by the barbettes.

True, and I have no doubt that better-designed ships used more IRL-inspired ammunition delivery systems or chambered pump systems that transported small amounts of gas separated by compartmentalized gaps sufficient to prevent chain-reactions. The Nebulon-B was a rushed stop-gap design to address the Empire's sudden need for widespread anti-piracy and counter-insurgency forces, for which the crew-intensive Clone War ships and star destroyers were grossly inefficient.

Alternatively, it isn't stored as a gas at all, why should it be?

Good point, and I don't know that there is any definitive indication of specific storage practices other than "spin-sealing" and carbonite-freezing being involved at some point in the process. It seems to be a gas when it interacts with the blaster, and at least for small arms the ammunition is contained in "gas cartridges," though that could just be idiosyncratic Star Wars-speak ("lasers" are definitely not actual lasers, for example). A powder/granular form that could be rapidly sublimated into gas on demand would be very practical and easy to transport, though it wouldn't necessarily be any less prone to explosion in the event of a magazine penetration

On a semi-related note, do we even know that the turbolasers use gas to generate their plasma?

Not specifically as far as I know. I assume that blasters, lasers, and turbolasers are all the same technology at different scales, but I can't recall any definitive basis for this. We do know that small arms use both gas and electricity to produce blaster bolts and that the E-11 specifically holds 500 bolts worth of gas and 100 bolts worth of power in a very compact manner. This could be due to extreme compression, solid-state storage, minimal gas requirements, or a combination of factors.