MULTIPLE POTENTIAL CONFLICTS AND FREQ CONGESTION PLAGUES A B737-800 FLC DURING A NON RADAR DEP PROC FROM TNCN; FO.

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DEV FROM ATC CLRNC UNDER CAPT'S EMER AUTH. ACFT HAD LEVELED OFF AT CRUISE. NON RADAR DEP CTLR ADVISED US OF A VFR AIRPLANE 12 O'CLOCK POS; 500 FT BELOW; 3 MI. TCASII SHOWED ANOTHER ACFT (ATR42 OR SIMILAR) AT 12 O'CLOCK POS; 3.5 MI IN A HIGH RATE OF DSCNT TOWARDS US. SINCE I COULD NOT DSND TOWARDS THE VFR BELOW (THIS ACFT MAY HAVE BEEN ON A VFR PLAN; BUT HE WAS IN THE CLOUDS) I ELECTED TO MAKE A R TURN. AS SOON AS WE SAW THE DSNDING IFR LEVEL LESS THAN 500 FT ABOVE US; AND PASS WITHIN .5 MI; WE RETURNED TO COURSE AND ADVISED ATC. THIS CTLR WAS TASKED WITH CLRNC DELIVERY; GND TWR AND APCH AND DEP CTL. HE WAS SEVERELY OVERLOADED -- NOT HIS FAULT. POST EVENT: SXM ATC VERY INCENSED WE HAD TAKEN EVASIVE ACTION. CAPT PROVIDED NAME; AND THAT I TURNED THE ACFT UNDER EMER AUTH. CTLR OVERWORKED BEYOND HIS CAPACITY -- NON RADAR ENVIRONMENT. WITH TFC THIS DENSE; VERY DANGEROUS SIT. MANNING LEVEL OFF; ST MARTEN TWR TOTALLY INADEQUATE. PRIOR TO OUR DEP; THIS CTLR HAD AN AIRBUS A340 AND A B767 CIRCLING AT THE SAME ALT IN CLOSE PROX TO EACH OTHER OFF THE W END OF RWY 9 AT SXM. FOR THIS CTLR; A NON RADAR ENVIRONMENT MEANT CONSTANTLY MAKING POS QUERIES OF ALL ACFT. THIS CAUSED BAD FREQ CONGESTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: TCASII RA GAVE A DSCNT COMMAND AS THE LOWER TFC WAS NOT DETECTED BY TCASII. THE RPTR'S TCASII SHOWED 2 ACFT; BUT NO ALT ON ACFT Y; THE VFR TFC. THE VFR TFC WAS IMC IN VFR FLT; NOT VISIBLE AND HAD NO MODE C; IT NOT BEING 'REQUIRED' IN THAT AREA. AS TO FLT Z; THE CTLR REASONED THAT THE TURN MADE BY THE RPTR WAS NOT REQUIRED AS HE EXPECTED THE ATR Z TO LEVEL OFF AT THE ASSIGNED ALT. THAT ATR FLT WAS CREWED BY A FOREIGN FLC WITH A HVY ACCENT. RPTR STATED THAT A YR AGO A COMPANY FLT HAD A VERY CLOSE NMAC DURING THE DEP PROC WITH ANOTHER ACFT. IN A FEW DAYS THE COMPANY IS HOLDING A MEETING WITH THEIR SAFETY DEPT AND THE FAA TO DISCUSS THIS ONGOING SIT. THERE IS NO RADAR; THE CTLR IS HANDLING MULTIPLE POS AND THE FREQ CONGESTION FROM AT LEAST 10 ACFT WAS INSUFFERABLE. CTLR WAS 'GUESSING' AS TO OTHER ACFT POS; HAVING TO CONSTANTLY REQUEST POS OF RESPECTIVE ACFT.