Keyboard input handling at that level generally isn’t character-based, and instead requires looking at scancodes and modifier keys, and sometimes also distinguishing between keyup and keydown events.
You generally also don’t want to produce different Unicode sequences depending on whether you have an “é” key you can press or have to use a dead-key “’”.
I guess your conception is that one could then define
<dead_acute> : <combining_acute_accent>
If you want to define such a key, you can probably still do it, you’ll just have to press it in the opposite order and use backspace if you want to cancel it.
The fact that dead keys happen to be prefix is in principle arbitrary, they could as well be suffix. On physical typewriters, suffix was more customary I think, i.e. you’d backspace over the character you want to accent and type the accent on top of it. To obtain just the accent, you combine it with Space either way.
Depends on the system. X11/Wayland do it at a higher level where you have `<dead_acute> <e> : eacute` and keysyms are effectively a superset of Unicode with prefix combiners. (This can lead to weirdness since the choice of Compose rules is orthogonal to the choice of keyboard layout.)