A320 FLC EXECUTES GAR AT SEA DUE TO TFC DEPARTING ON RWY AND S46 SEQUENCING.

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VECTOR TO FINAL APCH WAS BUSY. NEVER ADVISED US OF WHICH RWY. WE MADE ASSUMPTION OF RWY 34L AS CALLED BY ATIS. WE ADVISED ARPT IN SIGHT; APCH CTLR CLRED US FOR VISUAL TO RWY 34R. HE ADVISED US OF DASH 8 BEHIND US WHO HAD US IN SIGHT GOING TO RWY 34L. AT 900 FT WE GOT AN RA TO DSND AT 1900 FPM. AFTER WE CLRED CONFLICT; WE GOT RE-ESTABLISHED ON GS. AT 400 FT THE TWR CLRED AN ACR B757 FOR TKOF ON RWY 34R. WE WERE ABOUT TO FLARE. ACFT STILL ON ACTIVE SO WE INITIATED GAR. AS WE WERE VECTORED TO FINAL; APCH CTLR ONCE AGAIN NEVER CLARIFIED LNDG RWY UNTIL WE WERE ON BASE CLRED FOR VISUAL FOR RWY 34L. OVERALL; SEA APCH AND TWR APPEARED TOTALLY OVERWORKED AND UNORGANIZED. TWR REQUESTED A CALL UPON LNDG. I SPOKE WITH TWR SUPVR AND ADVISED HIM OF ENTIRE INCIDENT. SUPPLEMENTAL INFO FROM ACN 515560: ACR WAS STILL ON TKOF ROLL AND WE HAD TO EXECUTE A MISSED APCH. TWR WANTED US TO MAINTAIN VISUAL SEPARATION WHICH WAS IMPOSSIBLE TO DO SINCE WE HAD A NOSE HIGH ATTITUDE. WE ADVISED TWR AND TURNED 30 DEGS R. ALSO; TWR CHANGED MISSED APCH ALT DOWN TO 2000 FT. VERY HIGH WORKLOAD.