My thoughts are there are just too many options in LayerFX for a single dialog. I'm not sure it makes sense to combine everything into a single dialog. As far as the operations being applied to the image in realtime, that would be the goal of GEGL. I have not compiled GIMP 2.9.x from GIT in a while, so I'm not sure how far along they are in converting to GEGL. About half the GEGL operations don't work correctly in V2.8.14 (at least for me).

If you'd like to test the current development version, there are nightly builds of GIMP 2.9.x on Dark Refraction. Partha probably has a fairly stable build of GIMP 2.9 on his site, as well.

It could be done with a "notebook" type of dialog (as used in "Map Object", "Lighting Effects" and various other filters). But there is a lot of work involved to re-structure the LayerFx plugin that way and the resultant dialog would be rather large!
I don't really understand what you mean. All the LayerFx filters do already provide an "in-situ" preview (in the image window) ..you just have to enable it with the "Preview" checkbox (at bottom left in the dialog).