This thread is more about editing layers and not about modify Squeekboard entirely. And some of your wishes are things that I think will never be supported with Squeekboard and need a totally different solution. Squeekbaord has predefined keys an cannot simulate everything. This may gets better if shortcuts would be implemented (this way we could implement up + left arrow to diagonal up-left arrow or even up-up-left).
Best thing would be if games support it by themselves. An additional keyboard is just a poor fix for unsupported inputs. However, in some cases it makes sense.
@ 5.) I created a new thread for that specific topic. Even if it could be handled in this thread, it’s better to make an own, because it’s for a very specific layout that will need a lot of work.