Ok, now I got you completely.
No, sorry, expressions cannot be used as constants in the parameter mapping.
If you cannot resolve the issue on the client side (setting the a default value for a parameter and map it to the underlying view) I see just one more or less acceptable solution if you have to do it on view level.
Instead of copying the whole view X1 I would introduce some lightweight view which is doing nothing else than defining the parameter with the default value (via expression) and consuming the X1 view.
Y1 (view w/o parameter)
-> D1 (view which has parameter with default value)
-> X1 (mapping of parameter of D1 to key date parameter)
Y2 (view w/o parameter)
-> D2 (view which has parameter with another default value)
-> X1 (mapping of parameter of D1 to key date parameter)
It is not a nice solution, but if you do not to much different default values the number of "default views" should be manageable (especially because the full blown logic is defined only once in X1).
But of course there are some drawbacks, especially if you extend view X1 with additional fields, you have to adjust the views on top of it also if necessary.
Regards,
Florian