I thought the left Windows key normally was mapped to Meta.
No, normally the Windows keys are mapped to Super. On a normal keyboard there is no Meta.
Few questions.
Meta is the mod_switch key and replaces AltGr?
Super_L under right shift is the windows key?
What is the Pyra menu key then if it isn't Super_L? Super_R?
Why not keep Compose mode as Shift + Enter?
Why not use the vacant but available Hyper_L and Hyper_R as modifiers for only the symbols instead of remapping established AltGr and Meta?
Most keyboard either have two Alt keys or one Alt key and one AltGr key. Since I want both Alt and the Level3 modifier (Fn/AltGr) available both on the keyboard itself and on the shoulder buttons, it makes sense to call the latter Meta (or Hyper, that would work too) so we can have Alt_L, Alt_R, Meta_L and Meta_R that can all be nicely distinguished from one another.
In my layout, the right shift key on the keyboard would actually be Super_L (or Super_R, it does not really matter), which would be mapped to Shift by default but which would be a good candidate to map to Compose (the default Compose could be Meta+Super, and perhaps Shift+Enter too for backwards compatibility, although I don't really like that since there might be applications that actually use Shift+Enter). The actual Shift_R key is the L1 shoulder button like on the Pandora. A dedicated compose button obviously has the benefit of less keypresses.
The Pyra button would map to the Menu key like on the Pandora. It could also be mapped to Super, that would also make sense. But I'd rather have a Super key and a Menu key instead of two Super keys, so if the Pyra button becomes Super, then the key below Enter (right shift) should be Menu (remapped to shift by default).
Meta and Hyper are both vacant but available modifiers; I prefer Meta because it is one letter shorter and sounds better to me (e.g. it corresponds better to a button that can be used to toggle system settings like wifi/3g/bt/hdmi), it and there is a nice established symbol for it (◆). Also, afaik there are no existing (historical) keyboards that have a Hyper key but no Meta key, while keyboards with Meta keys and without Hyper keys are relatively common: