>> 2. Create a unified way to repeat the last command with its argument (à 
>> la "." command in Vi), by binding the existing "repeat" command to a 
>> single key binding, for example "C-=".  With this it also becomes 
>> possible to change the argument between two repetitions, possibly to 
>> something different than "opposite".  No command must be changed.
>
> Is that different from C-x z?
>
> It could be another binding for C-x z.  Perhaps C-= is moreconvenient.
>

It is different indeed.  The OP's problem was to minimize key presses and 
releases to "reduce the probability of RSI".  With C-= bound to repeat, 
you can change the command argument between two repetitions of a command, 
without typing the command again and without even releasing the control 
key.  You cannot do that with C-x z.

An example: calling (other-window 1) four times followed by (other-window 
-1) one time requires:

C-x o C-x z z z C-- C-x z : nine keys pressed, control key pressed and 
released three times

C-x o C-= C-= C-= C-- C-= : seven keys pressed, control key pressed twice 
and released pressed only once