_Flashing a different year model caibration id wil not usually cause car not to start but it may cause some issues talking to say dash cluster, i know this first hand.
If the battery is good and the key symblol on dash is going green thats good.
If the dealer can talk to the ecu and extract the calibration id and the check sum ie cvn than the ecu is communicating with obd port.
if the dealer cannot flash the ecu or extract the rom\tune then its likey been flashed with brzedit or ecutek or the ecu actually has problems.
If the dealer can read the cvn (checksum) and its different to what a stock rom calid cvn is then your stuffed they have proof its tuned
if you dont have diagnostic tools then it going to get difficult
even an obd scantool will give you a clue if their are any storred error codes..
maybe get a subaru auto electrician to look at it it might be a simple fix
the autoelectrician should be able to swap in a same year\model ecu see here
http://www.ft86club.com/forums/showthread.php?t=102810