Uhhh lemme revise what I said because I was wrong.
Yes, it is legal. However, video game companies will do anything in their power to stop the production of emulators, even if the emulator requires an original game disk to play the game. A famous example of this was back when emulators came on disks and this company made a playstation emulator required you to use the original game disk to play it. Sony still got on their ass about it and not only ended up making them stop the production of the emulator but they also led the business's eventual bankruptcy.
I have not seen any big lawsuits, or throwaway suits, in recent times and "substantial non infringing uses" would seem to cover the rest. Emulators are up on big sites, discussion is very much public, said companies persuaded ebay, amazon, paypal and co to suspend payments to all sorts of things like flash cart development but any number of them have donate buttons, or things like patreon running.
Likewise that is a bit of a glossing over of Sony vs Bleem and the others around that time
maybe watch it at 1.5 speed, not the biggest fan of the video but covers many of the points involved
As for the matter at hand it is unclear. On the face of it if you are not using the ROM and the cart at the same time; can't pop say a megadrive game in your megadrive and use a dump of said same at the same time such that you are playing two copies of the game, this is more established with rules on how backups should work than emulation but I would look there were I in court and arguing. However then if we are imagining we are in the US for this then the DMCA rears its head and an exemption for bypassing technological protections has not been established so much for game consoles yet (almost everything else like it -- tvs, phones, cars, routers... but game consoles seem to be rather absent) so I don't know what goes there.
On the question of the OP if I take a more literal reading then emulators are just code and I can use VBA all day long to play my homebrews, even right outside Nintendo's offices. The intent was clear though so I will leave that one.
Regarding downloading vs making a copy yourself then nobody really ever established harm done. If you are caught torrenting, or using a similar p2p protocol, it then you might be guilty of distribution by virtue of uploading it as part of it all -- there is a reason every torrent case you see will tend to have uploading as the main charge.
In the end I still don't know but if you own a copy of the game at the time of playing, and the copy of the game is not being used elsewhere, then it is about as close to the line as you can get. I doubt a police officer would arrest you (if it is even a non civil affair) and I doubt a company would risk a court case if it came to pass that the previously mentioned stuff is what you are doing, if for no other reason than it could risk some precedents being set that they are quite happy to have a bit unclear at present (mainly as all logic says it is not likely to end in their favour).
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