Remap keys including a modifier

I use an alternative german keyboard layout on Ubuntu 18.04:

~$ setxkbmap de koy

Now I want to swap three keys (Mod3 → f → ß → Mod3) as shown in that picture:

This are the keycodes for these keys:

~$ xmodmap -pke | grep -E 'keycode  (33|48|51)'
keycode  33 = ssharp U1E9E ssharp U1E9E ampersand Greek_finalsmallsigma KP_Add KP_Add jot NoSymbol ampersand Greek_finalsmallsigma KP_Add KP_Add jot
keycode  48 = f F f F at Greek_phi period KP_Decimal Greek_PHI NoSymbol at Greek_phi period KP_Decimal Greek_PHI
keycode  51 = ISO_Level3_Shift NoSymbol ISO_Level3_Shift

Q1: Why are there 15 columns for the f key and the ß key? /usr/share/X11/xkb/symbols/de only defines 8 columns!

I can swap the keys by using these xmodmap commands:

~$ xmodmap -e "keycode  51 = ssharp U1E9E ssharp U1E9E ampersand Greek_finalsmallsigma KP_Add KP_Add jot NoSymbol ampersand Greek_finalsmallsigma KP_Add KP_Add jot"
~$ xmodmap -e "keycode  33 = f F f F at Greek_phi period KP_Decimal Greek_PHI NoSymbol at Greek_phi period KP_Decimal Greek_PHI"
~$ xmodmap -e "keycode  48 = ISO_Level3_Shift NoSymbol ISO_Level3_Shift"

Now the keys are swapped but …

• the ß (ssharp) key returns ß on all levels! Expected (as before swapping): ß ẞ & + ς ∘
• key 48 (former f key) works as Mod3 except when used together with other modifiers: Shift+Mod3+a should give an α but gives an A now.

Just checking again:

~$ xmodmap -pke | grep -E 'keycode  (33|48|51)'
keycode  33 = f F f F ampersand Greek_phi KP_Add KP_Add Greek_PHI NoSymbol ampersand Greek_phi KP_Add KP_Add Greek_PHI NoSymbol f F ampersand Greek_phi KP_Add KP_Add Greek_PHI NoSymbol f F ampersand Greek_phi KP_Add KP_Add Greek_PHI
keycode  48 = ISO_Level3_Shift NoSymbol ISO_Level3_Shift NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol ISO_Level3_Shift NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol NoSymbol ISO_Level3_Shift
keycode  51 = ssharp NoSymbol ssharp U1E9E at Greek_finalsmallsigma period KP_Decimal

keycodes 48 and 51 are not as I set them.

Q2: What did go wrong?

In some other questions on StackExchange people say that it's important to remove or clear modifiers before changing them. But this always fails:

~$ xmodmap -pm
xmodmap:  up to 3 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Shift_L (0x32)
control     Control_L (0x25),  Control_R (0x69)
mod1        Alt_L (0x40),  Meta_L (0xcd)
mod2        BadKey (0xcf)
mod3        ISO_Level5_Shift (0xcb)
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce)
mod5        ISO_Level3_Shift (0x5c)

~$ xmodmap -e "remove mod5 = ISO_Level3_Shift"
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  118 (X_SetModifierMapping)
  Value in failed request:  0x17
  Serial number of failed request:  11
  Current serial number in output stream:  11

~$ xmodmap -e "clear mod5"
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  118 (X_SetModifierMapping)
  Value in failed request:  0x17
  Serial number of failed request:  8
  Current serial number in output stream:  8

Q3: What do these errors mean?