Alicornutopia

It is a form of mental sorcery. In order for any form of mental sorcery to work you need to know the person really well, like close contact for a couple hundred years minimum, while they are totally or almost totally free to act as they wish - even small numbers of orders constraining their behavior in ways they wouldn't have chosen can throw off your understanding of how their mind works too much.

If I'm not mistaken, sorcery can transmute stuff. How long does it take to learn that? Can it only transmute metals or can it do other things, like precious stones?

You can transmute stones too, or kinds of wood, or liquids, or pretty much whatever. It requires more staring-at-your-thing than most spells but it's not actually that hard to learn to do at all.

Do orders work remotely, through, say, speakers or video or some kind of voice projection sorcery? What about lip reading or sign language?

For that matter, does voice projection sorcery exist? Is there a range limit for sorcery? Can you use, for instance, binoculars or a telescope to sorcer stuff very far away? Can you use video feeds?

Orders work through technological or magical audio or telepathic relays provided they're in real time and are not converted or sent through another agent's interpretation in between. Lip reading doesn't work, sign language does.

Voice projection sorcery exists but is hard to get to work over substantial distances or outside your own turf. There is not a fundamental range limit but your ability to get accurate information about all the factors drops off; narrowing your info to a visual channel is typically not going to get you anywhere. You can use a video feed as long as you also know where you are located relative to its subject and can do whatever you're doing on pure flattened audiovisual, which is very dubious.

Could a Materia god kill a fairy?

Could you do a gate on pure flattened audiovisual?

Aren’t Materia gods more powerful than Ruin? If so, it seems likely one could kill a fairy.

I would guess that they could send a fairy to the Materia afterlife, and that it wouldn't matter very much whether the mechanism was actual death or something more like planar transport.

Materia doesn't support outright obliteration of sapients. And a Materia god outside of Materia wouldn't be able to do anything cheaty at all.

I don't know what "do a gate on a pure flattened audiovisual" means but probably not.