If on terminal leave & applying for ROTC, will having signed a declination statement (for assignment after my ETS) disqualify me from ROTC?

Seems strange that you were placed on assignment after ETS, make no sense. EDAS shows that you do not have enough time to be placed on assignment, and you cannot be placed on assignment within 90 days of your ETS. you might find the answer to your question in AR 601-280, or AR 350-200. If you signed a DEC statement, you are telling the Army that you do not wish to follow orders. You probably will require a waiver or determination before you can get commissioned.