I am not sure I follow- the basic DS menu should handle most editing and should you have to force something you have a lot more problems on your hands (I don't think there are any forbidden names). All most programs will ever touch is wifi settings though you might look at DSOrganize (I am pretty sure it only had wifi plus some bonus wifi settings storage but it has been years).
If you are thinking about doing it on a per game basis then it is probably easier to make a cheat to do it (although anything can theoretically access the firmware a lot of the time the settings of it will be copied to memory at boot and operated on from there- it is how a lot of the language forcing cheats work).
If you have a file you dumped and want to do it on a PC then Crystaltile2 will be able to handle it (various places but under the tools pulldown menu is the main one). That said it is usually just as easy to click "config- firmware settings" on desmume.
If you mean you want the extra characters in your language ( ØæÅ?) then it is not going to happen I am afraid and even if you somehow managed it then it would only be for your DS and I would be not surprised in the slightest to see all manner of games crash and start crying should they have to try handling it- the DS firmware has a font built in and that is what it uses for firmware things. Games can (and often do) have other fonts but should something be wrong from the firmware level then things get can get just as messy.
As for the second question actually I am not sure what there is for a soft DS firmware flash (hardware is sorted by ppflash- http://www.darkfader.net/ds/ ) as nobody in the general public as it were really ever had cause to do it*- the obvious thing to look at would be either the flashme installer or the FWNitro installer to see if you could piggyback onto that but I am not sure what goes there.
*if you were developing a firmware (to my knowledge all of three different ones have ever been made) then you already know more than is necessary and the only other thing would have been parasyte to work around Nintendo banning him.
Checksum failures........ never thought about that.I think he wanted to make a 10 character name vs a 5 character name. I guess it could be possible if you knew what address to write the data to and if it even writable or it will even save I don't know. then there is also the risks of the ds no longer booting up because of some checksum calculation failure or some kind of buffer overflow caused by the extra long sting.
The website has a download of the firmware.nds (you have to delete the last folder in the file tree and select the "DS stuff" folder. There you can select "DS Firmware".As for the soft flash I forgot about NoFlashMe (should be available from http://home.comcast.net/~olimar/flashme/ )- it should have a firmware inbuilt which you can then piggyback on (I recall some people trying things there one day) rather than the others which are patching style programs.