Can you boot into OFW at all, without SD card inserted? I'm not sure but you should be able to boot into OFW if you press the Power button while holding down both volume buttons. If it works, you can start from scratch with either
sysNAND or
emuNAND CFW, depending on which one you'd like to use. That's what I'd do anyway, but note that I have no experience with hardmodded Switches.
to play Switch games, I would just place the SD card into my computer, copy-paste the XCI rom into it, and then place the SD card back into the console. where will these games be displayed on the Switch?
Not really. First of all, if you SD card is FAT32, most of the XCI files won't fit onto it due to the FAT32 filesize limit. Instead you're supposed to install your games with one of the installers. I suggest using DBI as it always worked for me from the very first day.
Here's a good video on how to use it. Note that the latest EN release is
658, while the latest RU only release is
678. Basically, once you properly install the drivers for the DBI on your PC and it gets recognized, you should be able to run the dbibackend.exe and connect your Switch to the PC via USB cable. It's should be pretty simple.
While some other users may prefer Tinfoil due to its free shops, I was never a fan of it since DBI did the job and I could easily find some truested sources for XCI and NSP files on the Internet that we aren't allowed to link to here due to anti-piracy reasons.
to run mGBA/melonDS emulators I just copy-paste the NRO files (+NAND/BIOS files for the DS) into the SD card, correct?
I'm yet to use emulators on my Switch, but if they are in NRO format, they should go into switch folder on the root of your SD cards as that's where all the apps go usually. As for the roms themselves, you'll have to check for yourself. There's probaby a guide or two, probably inside the app as well once you run it. Maybe you shoud check
Retroarch as well?
let's say I downloaded Super Mario Bros Wonder XCI Rom. do I need Sys-patch to play the game? I don't really understand what Sys-patch is in general.
Yes. sys-patch does the same thing sigpatches do, allowing you to play game backups while bypassing any and all signature checks which would render those games unplayable otherwise. These days, I prefer using sys-patch instead of sigpatches as it's much easier to update and usually doesn't need to be updated on every firmware revision.
go online with a modded console?
Access homebrew apps, play in local multiplayer, use LAN play maybe? Download amiibo data, some people even use all-in-on Switch updaters (which I don't personally recommned as I find updating manuallly much easier in a long term).
install Android (assuming you have a smartphone)?
Because they can?

I'm not using Android nor Linux on my Switch as I don't need it, but I've seen cases where some games run better on Linux build of a Yuzu emulator for the Switch than on a regular Switch. Maybe they want to run Android games? I don't know, but there are probably some good use cases. Also, using Android on Switch and going online won't get your Switch banned either (as far as I know), so there's that.
install tinfoil (assuming you have a PC)?
Free shops, I guess. Even paid shops, but I'm not sure how well those work. I'm perfectly fine finding the stuff I need the manual way and installing whatever I need via DBI.
To play the latest titles that require it?
is there a possibility to be banned by just doing the abovementioned use, without ever going online?
Here's how to avoid getting your console banned. Check the Best Way to Avoid Bans from
this thread as well.
@Draxzelex made a pretty comprehensive thread about anything related to Switch bans.
In any case, if you ever install e.g. a pirated game on your sysNAND, you are effectivly making it "dirty" i.e. it's no longer "clean". Going online with dirty sysNAND will either get your console banned or flagged to be banned sometime later. Having a dirty sysNAND and not going online is pretty much delaying the inevitable.
If for some reason you want to go online, play official games and use eShop without a problem, make sure to keep your hacking activities to your emuNAND and make sure that you're blocking Nintendo servers by using 90DNS or DNS MITM. Some people say that even blanking your prodinfo is a good thing, just make sure to keep your prodinfo backups safe. Reading from sysNAND is fine, writing anything to it the unofficial way probably isn't.
I've also wrote a short guide on updating your Switch which you can find
here.