as i see it you have 2 options:
1.to use set analysis in all expressions in the sheet , that way the user won't be able to change anything
but this method mean you need a lot of maintenence.
2. to create a trigger that upon activating the sheet
will select current month, and then lock the moth field
when the user leaves the sheet you need to unlock the field
in this method there is no maintenence
but advanced user can unlock the field and change the selections
