VFR IN MARGINAL IFR; TRACK DEVIATION ON APCH FOLLOWED BY MISSED APCH.

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STOPPED AT JACKSON; MS; FOR FUEL. OBTAINED WX BRIEFING FOR NEXT LEG; JACKSON TO FT SMITH. WX AT FT SMITH 1600' BROKEN WITH LIGHT; INTERMITTENT RAIN; 4 MI VIS. WE ELECTED TO CONTINUE VFR ON A DIRECT COURSE BY LORAN. APPROX HALF WAY WE ENCOUNTERED LOWERING CEILINGS AND RAIN SHOWERS. CEILINGS APPROX 2000'; VIS 3-6 MI. WE CONTINUED VFR; TWICE SKIRTING HEAVY RAIN. WHEN OUR DME AND LORAN READ 7 MI FROM FSM VOR WE ENCOUNTERED HEAVY RAIN; TURB AND PATCHY FOG. A CFII PLT WAS IN RIGHT SEAT. WE ELECTED TO CALL FOR AN INSTRUMENT APCH. WE RECEIVED A BACK COURSE APCH AND WERE VECTORED TO THE FINAL APCH FIX. THE CFII WAS NOT FAMILIAR WITH CTL RESPONSE OF THE SMA; PLUS WE WERE HAVING COM PROBS. WE BROKE OUT TOO FAR OFF FLT PATH TO MAKE A SAFE LNDG. I TOOK THE CTLS; KEEPING THE RWY IN SIGHT; FLEW ALONG SIDE THE RWY; MADE A DOWNWIND SHORT BASE AND LANDED. APPROX 1 HR AFTER LNDG THE CEILING WENT BACK TO 1500' AND VIS IMPROVED AND IT STOPPED RAINING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATED THAT EVEN THOUGH HEY ENCOUNTERED RAIN; THE CONDITIONS NEVER WENT BELOW VFR LIMITS AND THEY STAYED CLR UNTIL HAD CLRNC FOR THE APCH AT FSM. THAT WAS FIRST THEY GOT INTO CONDITIONS BELOW VFR. THE MISSED APCH WAS CAUSED BY THE CFII APPARENTLY FORGETTING HE WAS ON BACK COURSE APCH AND CORRECTED WRONG DIRECTION TO CENTER LOC. WHEN SAW OUT OF POS TO LAND; RECEIVED TWR CLRNC FOR DOWNWIND AND VISUAL APCH SO WOULD NOT HAVE TO MAKE FULL MISSED APCH PROC.