B777-200 ENTERED INCORRECT ALT INTO FMS AND FAILED TO MEET CROSSING RESTRICTION ON STAR.

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CREW: RATED CAPT IN L SEAT MYSELF IN R SEAT. WE WERE ON THE SEAVU ONE ARR INTO LAX APCHING KONZL INTXN WITH REQUIRED 280 KTS/17000 FT IN THE FMS. WE WERE TOLD TO CALL SOCAL APCH AND SLOW TO 250 KTS AND DSND VIA THE PROFILE. THE CAPT WHO WAS FLYING DID NOT SET THE ALT WINDOW FOR 7000 FT WHICH IS THE END OF THE ARR. WHAT HE DID DO WAS TYPE 250 IN THE FMS. AS HE DIDN'T PUT 250; THE FMS THOUGHT IT WAS AN ALT AND DISPLAYED FL250 WHICH HE ACTIVATED. I TOLD HIM THAT WAS INCORRECT. HE DELETED IT. I ALSO TOLD HIM TO SET 7000 FT IN THE ALT WINDOW. NOW; I WAS IN THE MIDDLE OF TALKING TO APCH AND RUNNING THE DSCNT CHKLIST. THE FIRST THING I NOTICED WAS OUR SPD ACCELERATING THROUGH 290 KTS (ASSIGNED 250 KTS). I MENTIONED IT. THEN I NOTED WE WERE BUSTING OUR ALT AT TRTLE AT A FAIRLY GOOD RATE OF DSCNT. I MENTIONED THIS AND WHEN TIMELY ACTION WASN'T TAKEN. I TOLD HIM TO TAKE IT OFF AUTOPLT AND LEVEL AND SLOW. I BELIEVE WE PASSED TRTLE BELOW 16000 FT; I SAW 14800 FT BUT WE MIGHT HAVE PASSED TRTLE BY THEN. REST OF STAR WENT FINE. I BELIEVE THAT WHEN WE DELETED FL250; WHICH WAS MISTAKENLY ENTERED; WE DELETED THE AIRSPD AND XING RESTR. ACFT WANTED TO FLY THE PROGRAMMED DSCNT SPD OF 315 KTS. I WAS TASK SATURATED; DID NOT WANT TO LEAD HIM; AND THINGS HAPPENED FAST.