Turning the Escape key into a modifier that still acts as Escape when tapped

Question

I use a tiling window manager (xMonad at the moment), and due to this I need a key that acts as the ‘control key’ for window management. Whenever I use a keyboard with Windows/Super/Mod4 keys, I use the left Windows key as that key, while my right Windows key acts as Compose/Multi_key.

However, I also have a couple of ([very] old) keyboards that don't have any Windows keys (i.e. they're winkeyless), and there I don't have any key left that I can (comfortably) use as my ‘WM control key’. In the past, because xMonad allowed that, I used Control+Alt as my ‘WM control key (combination)’, but as I noticed after some time, this interferes with (some) Emacs (movement commands), so this isn't an option anymore. Therefore, I thought that I could utilise my Escape key in the following way:

• If I hold down Ecs, it acts like Mod4
• If I tap Esc, it acts as Escape

AFAIK, holding down Escape doesn't really do anything normally, so I thought it might be a good idea to do it like this. I then tried to achieve this by using xmodmap:

clear Mod4 
add Mod4 = Super_L Escape

What I expected: Super_L still acts as Super, and Escape is now an additional Super. The next step would have been to use xcape to get (some) of the old Escape key behaviour back.

What I got (the output of xmodmap):

xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):                                                                         

shift       Shift_L (0x32),  Shift_R (0x3e)                                                                                             
lock        Caps_Lock (0x42)                                                                                                            
control     Control_L (0x25),  Control_R (0x69)                                                                                         
mod1        Alt_L (0x40),  Meta_L (0xcd)                                                                                                
mod2        Num_Lock (0x4d),  Hyper_L (0xcf)                                                                                            
mod3        Scroll_Lock (0x4e),  Mode_switch (0xcb)                                                                                     
mod4        Escape (0x9),  y (0x34),  Super_L (0x85),  Super_L (0xce)                                                                   
mod5        ISO_Level3_Shift (0x5c) 

What I don't understand:

• Why is y suddenly set to Mod4, as well?
• Why does Escape still work as Escape (when pressed once) and Super (when held down; I noticed this when I tried to test my changes), although at this point I had not used xcape yet.

Is this a bug in xmodmap, or am I using xmodmap incorrectly? I suspect it's the latter, but could somebody help me with achieving what I'm trying to do?

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Further info (hopefully not irrelevant):

• My current OS is Arch Linux
• I'm open to solutions that use other software, such as xkb, xdotool, etc.
• I'm currently using the German QWERTZ layout (i.e. ISO, not ANSI), and sometimes Neo (which, AFAIK is implemented as another variant of the de keymap/layout; see output of localectl below)
• I want my solution to work keyboard/hardware independent: If I change my keyboard, I don't want to have to change any config files.
• Before anyone suggests this: Mapping Super to Capslock is not an option, as I sometimes use the Neo layout, which uses Capslock as one of its 3rd layer modifiers, and it's also used for the 5th and 6th layer. This also means that I'd like this solution to be keyboard layout independent, as well, so I can easily switch between, e.g. Neo and QWERTZ.
• I'm still open to other suggestions. My only demands: No/minimal interference with any other programs (e.g. Emacs or Vim), and the key (combination) to use as my ‘WM control key’ should be located on the left side of my keyboard, because most of the other keys I use to control my WM are on the right side of my keyboard (e.g. h, j, k, l,…).

Before any changes, i.e. my current situation, xmodmap outputs:

xmodmap:  up to 2 keys per modifier, (keycodes in parentheses):                                                                         

shift       Shift_L (0x32),  Shift_R (0x3e)                                                                                             
lock        Caps_Lock (0x42)                                                                                                            
control     Control_L (0x25),  Control_R (0x69)                                                                                         
mod1        Alt_L (0x40),  Meta_L (0xcd)                                                                                                
mod2        Num_Lock (0x4d),  Hyper_L (0xcf)                                                                                            
mod3        Scroll_Lock (0x4e),  Mode_switch (0xcb)                                                                                     
mod4        Super_L (0x85),  Super_L (0xce)                                                                                             
mod5        ISO_Level3_Shift (0x5c)  

My current .Xmodmap is:

add Mod3 = Scroll_Lock

And localectl says:

   System Locale: LANG=de_DE.UTF-8
                  LC_COLLATE=C
       VC Keymap: de-latin1-nodeadkeys
      X11 Layout: de,de
       X11 Model: pc105
     X11 Variant: nodeadkeys,neo
     X11 Options: compose:rwin,grp:ctrls_toggle

Answer 1

I use xcape for this. Map the key to Super_L in Xmodmap, and run xcape -e 'Super_L=Escape' when XMonad starts.