SA227 CREW AND BE1900 HAD LESS THAN LEGAL SEPARATION AT MYGF.

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AN IMMINENT COLLISION OCCURRED AS FREEPORT TWR WAS CTLING OUR ACFT AND A BE1900 TO LNDG AT FREEPORT. FREEPORT APCH CLRED US TO 1500 FT AND CLRED US FOR THE VOR RWY 24 APCH. SHORTLY AFTER; ATC XFERRED US TO TWR. WE ADVISED DSNDING THROUGH 2500 FT 1500 VR RWY 24 APCH. AT THIS POINT; THE TWR INSTRUCTIONS WERE CONFUSING. THEY ADVISED TO EXPECT TO CROSS OVER THE FIELD AND ENTER A R DOWNWIND. I ADVISED WE WERE NOT VISUAL; BUT WOULD LET THEM KNOW IF WE COULD ACCEPT THIS AS WE DSNDED TO 1500 FT. SHORTLY AFTER THIS; MY COPLT ADVISED TFC CAT 11 O'CLOCK; AT WHICH POINT WE SAW THE BE1900 DOING A CLBING R TURN TO AVOID US. WE BOTH WERE AT 1500 FT. EVIDENTLY THE BE1900 WAS ON A L DOWNWIND TO RWY 24 AND WE CROSSED PERPENDICULAR TO HIS FLT PATH. THE TWR NEVER ALERTED US TO THIS TFC; AND WE HAD NOT ACCEPTED A VISUAL APCH. THE CLOUD BASES WERE AT 1600 FT; AND RAGGED SO WE WERE STILL OPERATING ON THE INSTS. BETTER COM BTWN THE APCH CTLR AND TWR; AS WELL AS BETTER SEQUENCING AND ADHERING TO PROPER ATC PROCS WOULD HAVE PREVENTED THE SIT. I SPOKE WITH THE ATC SUPVR WHO ADMITTED THE TWR CTLR WAS A TRAINEE. BETTER SUPERVISION IS REQUIRED. FURTHERMORE; THE TWR CTLR EVIDENTLY THOUGHT WE WERE CLRED TO 2000 FT. THE BE1900 PLT ASKED WHAT ALT WE WERE ASSIGNED; AT WHICH POINT THE TWR CTLR SAID 2000 FT. TALKING WITH THE ATC SUPVR AFTER THE EVENT; HE SAID THE TWR CTLR WAS CONFUSED AND WE WERE CLRED TO 1500 FT. THE MARGINAL WX CONDITIONS CONTRIBUTED TO THE EVENT.