The Mustang I had its first victory (Fw 190) and first loss at Dieppe in August 1942, it was used in tactical recon because that was what it was best suited for, no other plane could do what it did.
First RAF Mustang Mk.I operational loss on 14 July 1942, aircraft of No.26 Squadron RAF flown by Pilot Officer H Taylor crashed at low level whilst strafing German troops near Le Touquet, France - either hit by ground fire or misjudged height during attack. (First non-operational RAF loss on 9 May 1942 in take-off accident.)
First RAF Mustang air to air combat victory, now known to predate that claimed during the Dieppe operation in August 1942. 'Kill' confirmed by RAF records declassified post-war, surviving Luftwaffe loss records and other records in Government Archives and dates from July 1942.
The P-51A / NA-73 was available, and due to it's excellent performance, they kept the project going, but they had some kind of problems with agility i still don't fully understand. I gather the P-51B saw a change in ailerons. All I can say is that the Allison P-51 did see action but it didn't really work out well in combat. If it had, I believe they would have made much wider use of it well before the Merlin-60 versions were available.
You obviously know little about the early Mustangs.
NA-73 North American Mustang Mk.I - no US 'P' designation as direct purchase contract between UK Government and NAA. Two aircraft from order provided to USAAC/USAAF for testing given XP-51 designation. 320 built.
NA-83 North American Mustang Mk.I - no US 'P' designation as direct purchase contract between UK Government and NAA. Minor changes and modifications identified in production of NA-73 implemented into NA-83. 300 built.
NA-91 North American P-51 Mustang, Mustang Mk.IA in RAF service. Lend Lease order for RAF, 150 aircraft. First with P-51 designation, no suffix. Basically similar to NA-83, but with change in armament to 4 x 20mm Hispano cannon. Number of these aircraft retained by USAAC/USAAF after Pearl Harbor and were first P-51 to enter US service.
NA-97 North American A-36A Mustang. First order officially for USAAF, 500 aircraft.
NA-99 North American P-51A Mustang, Mustang Mk.II in RAF service. Second order for USAAF, 310 built - originally was going to be more but production switched to Merlin engined P-51B/C.
As far as the RAF was concerned, the Allison engined Mustang worked out very well in combat, particularly in the role of a low altitude tactical reconnaissance fighter. In 1944 and 1945 in studies conducted by RAF staff in comparing the potential available aircraft for that role, they repeatedly came to the conclusion that the best possible replacement was more Allison engined Mustangs - not Hawker Typhoons, not various versions of Merlin or Griffon engined Spitfires, not Merlin engined Mustangs. Only problem was, they could not get them because the production lines had been fully switched to producing Merlin engined Mustangs for the 'pure' fighter role. Down low, with the engine modifications made by the RAF to the Allison engines in their Mustangs, they were fast, lighter than the Merlin Mustang, and had control responses that the pilots found more than matched their requirements. RAF, reluctantly, retired the last of the their Allison engined Tac/R Mustangs in August 1945 - they basically wore out the aircraft they had, combined with combat losses, usual training losses, etc. Pilots flying Mustang Mk.IA and Mk.II were still engaging in air to air combat - altho that was not their primary role - on an opportunity basis in and scoring 'kills' and 'damaged', confirmed, as late as February 1945 against types such as Me-262 and 'long nosed' FW-190s. They also racked up a very impressive score in strafing of ground targets, particularly railway steam engines and rolling stock, enemy transport, river barges and the like. Plus their best 'scores' were racked up by the Typhoon and Spitfire pilots, or the Squadrons of RAF or USAAF medium bombers who were called in as a result of their sighting reports on concentrations of enemy armour, transport, artillery positions and the like. RAF pilots in Allison engine Mustangs flew operations in them from 10 May 1942 until VE-Day, in May 1945, including first operation of single engine fighters based in UK against Western Germany in October 1942, first long range low level bomber escort against targets outside range of Spitfires and Hurricanes in September 1942, and a whole lot more.