G5 isn't really usable at all yet via Kha (Lubos recently added some code but it's still very early and the G5 API in Kore in not yet finalized). Also Metal/Vulkan is not about features, it's about manual resource management and multithreading. Same for G4/G5. feature-wise G5 will not provide anything G4 can't do as well and both of them can work on all target APIs. That said, G5 will of course work in Krom the moment it properly works in Kha.
Krom was primarily designed for Kha and to expose all system features (in particular all the things WebGL can not do) but you can also program it directly via JavaScript (but that's still a bit rough).

That happens because you don't actually use iMouse. It's consequently stripped out by OpenGL's shader compiler and then Kore can not find it. This doesn't prevent your program from working though, it's not an error message.

I don't know, what does "treat as a native build target" imply?

Sounds ok to me. Can you provide more information and maybe upload your code? What system do you use? Did you recompile your shader via node Kore/make?

Yes, that's expected. For fast build times just use the debuggers in Kode Studio. The Krom debugger allows code changes to be patched in at runtime but I need to fix it for current Haxe releases (on my todo list for end of this month).

Do something like addAssets("whatever", {quality: 0.9}) and Kha will on some systems use compressed textures. All compressed texture formats are lossy, that's why it isn't used by default - but even then Kha compresses the textures for faster loading and decompressed while loading. Kha always uses bitmap fonts for speed but the fonts are converted at runtime on first use so that any font size can be used.

Uhm, nothing important has changed since then and every existing backend is, like, a properly working and up to date example of a backend I suppose. But something like Magic Leap support doesn't even require its own backend, it's just another VR/AR device after all if I'm not mistaken. Look at the Rift/Vive/Hololens support in Kore.

The html5 backend is less aggressive in throwing out unnecessary data but that might change so be careful. It won't work at all in any of the native backends without readable set to true.

For non-rendertargets use the at function instead (we should probably unify that). But for that to work, the images have be be loaded with the readable option. When your assets are defined in your khafile you can just add that option in there ala project.addAssets("Whatever/**", { readable: true }); and if you load your images manually readable is a parameter of Assets.loadImageFromPath. Ideally you only use readable for images which really need it because it doubles RAM usage (images have to be kept in GPU mem for rendering and in CPU mem for direct access).

It's Kha/make unity. But I haven't updated the Unity backend in a long time, it's currently not working. Unsure whether it's worth it now that we have direct console support.