A CITATION 500 PLT; DEPARTING FROM LPR; AN UNCTLED FIELD; ON AN IFR CLRNC; EXPERIENCED 2 NMACS AND TOOK EVASIVE ACTION TO AVOID COLLISION; CAUSING HIM TO OVERSHOOT HIS ASSIGNED ALT.

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ATC CLRED US FOR DEP; HDG 360 DEGS TO MAINTAIN 3000 FT. COM ON CTAF REVEALED ONLY 1 CONFLICT; A TOUCH-AND-GO TO REMAIN IN PATTERN ON SHORT FINAL RWY 25. FOLLOWING DEP; TURNED R TO 360 DEGS AND BEGAN CLB AT APPROX 2000 FPM. NEARLY IMMEDIATE TCASII ALERT HEAD-ON WITH TARGET AT SAME ALT. SECOND ALERT SOUNDED (AEROSTAR) AS DSNDING ACFT ENTERED PATTERN (R TFC DOWNWIND; RWY 25). REACHING 3000 FT (CLRNC LIMIT) HAD CHOICE OF STRIKING FIRST ACFT (SLOW MOVING SINGLE); DSNDING FORWARD (NO TIME) DSNDING L (ANTENNAE FARM); ASCENDING R (JOINING BUSY PATTERN AT HIGH SPD) OR; CONTINUING CLB TO AVOID BOTH ACFT (OPTION CHOSEN). CLBED TO APPROX 4200 FT; TOLD ATC MY PREDICAMENT; DSNDED; WAS GIVEN 6000 FT. ALTIMETER SHOWED 6000 FT WHEN ATC SHOWED 6800 FT. CYCLED AIR SOURCE AND CYCLED PITOT HEAT AND CONFIRMED BREAKER SET. THEN; ALT SHOWED 5800 FT; WHEN ATC SHOWED 6000 FT. SUSPECT FROZEN PITOT (2 INCH SNOW FALL; SLUSH ON RWY DURING PREVIOUS NIGHT'S LNDG). ATC NEVER RPTED OPPOSITE DIRECTION ARRS FOR DOWNWIND R TFC RWY 25. LUCKY TO HAVE MISSED BOTH ACFT; BUT ATC COMPLAINED OF ALTDEV. RECOMMEND REVIEW OF ATC PRACTICES REGARDING THIS ARPT.