Somehow I believe, you're not aware of my slightly advanced version Generator:
http://ghostwhitecrab.de/generator/
"Not possible to read Game Genie codes in XXXX-XXXX format."
I've never used a real Game Genie cartridge but I believe this format issue is fixed in my version. The original didn't parse them properly so that this feature didn't work at all. I tested some cheats and they worked.
"After applying a Game Genie code, it's not possible to de-apply it."
That's not fixed. You'd have to remove the code and restart the emulation.
"No way of having emulation start by default when a ROM file is loaded."
Nobody ever asked for it. My version has a command-line switch "-a" which stands for "arcade mode". It goes to fullscreen mode and starts the ROM which you specified on the command-line immediately. If you're mostly playing a few games you could add
"generator-gtk -a /path/to/game" as menu item to your window manager. Auto-start on loading a ROM would be trivial if that's what you want.
"I have problems with SDL - namely, that it blocks all my other audio sources. Only ALSA and ESD work fine (most of the time), ALSA more so than ESD. I'd love to have ALSA support."
Simple. Put this line into your ~/.profile or ~/.bash_profile or ~/.bashrc whatever shell and file you have:
SDL_AUDIODRIVER=alsa; export SDL_AUDIODRIVER
As budgie wrote, you have to compile my patched Generator with --with-sdl-audio. Otherwise, it might pick-up some other audio method.
"Haven't yet discovered fullscreen mode, and I'm starting to suspect it doesn't exist."
Use Control+F or Emulation->View->Fullscreen to toggle fullscreen mode.
"I don't want to use Gens because:
1- They don't offer precompiled binaries for GNU/Linux, unlike Generator."
This is simply a PITA especially for a single person or small teams because there are so many platforms and options to compile a program. Also binary packages are constantly broken if not shipped by whatever package management or distribution you use. For performance-sensitive apps like Generator which benefit a lot from compiler switches like --march it's also not very cool to either provide 20 different binaries or otherwise some half-optimized that works sub-optimal.
Several distros provide binaries for Generator as well as my version. That's the proper source to look for binaries. Even if people would contribute binaries I wouldn't like to distribute them on my website because it's impossible to check whether they added trojan horse for example. A non-evil issue is a badly compiled binary like ignoring serious compile warnings or half-assed attempts to fix some compile etc.
The source tarballs I provide are signed, so as long as you check the signature you only have to trust me and the previous developers. But as said, it's available in distros so why would anyone take this risk and if they don't care they can probably find a binary with Google...
"2- Compiling it from source is a pain in the rectum - at least with the last GCC version."
Well, for Generator it should be trivial. You have to know a few basics of course. Basically just "./configure && make && make install". Otherwise, look whether your distro ships it and if not ask them to add it. Or just ask the developers or in their forums. It's just said that many Linux distros make it much more difficult to compile things yourself than it actually is.