The fighter-bombers wouldn't launch an attack unless they thought they could hit - attacks were very dangerous due to the very effective German light flak. In fact, that flak in distracting the pilots may well have contributed to the problems in making accurate rocket attacks. This extract from 'Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45' explains the problems with the British RP, but in principle any rocket suffered from these to a greater or lesser extent:
"This fall in accuracy experienced in action may be attributed to the curious trajectory of the RP, which first dropped below the line of sight and then accelerated as the rocket motor took effect before it dropped again. Because of this it was generally desirable to fire them at a range of between 900-1,800 m. They were also very susceptible to side winds, with a mere 15 km/h wind being enough to miss the aiming mark by nearly 5 m, and the aircraft had to be absolutely steady at the instant of launching. This meant that a pilot needed a very cool and calculating head to ensure reasonable accuracy, something that was difficult to achieve in the heat of battle. It is worth noting that high-velocity cannon did not suffer from this problem, so would have experienced a much less significant fall-off in accuracy under combat conditions. "
I don't know of any 3.5 inch rocket. The British RPs were known as 60 pdr or 25 pdr Rocket Projectiles, depending on the warhead, and used a 3 inch diameter motor. There were two USAAF rockets: the 4.5 inch and the 5 inch HVAR. This is the info in FG:WW2.
"The two RPs used by the USAAF had different histories. The 4.5" version originated from a 1940 request from the Ordnance Department to the National Defense Research Committee for help in developing a rocket primarily for use in aircraft. The availability of British research speeded the task so the first prototype was tested at the end of 1941. Somewhat bizarrely, the calibre of 4.5" (114 mm) was determined broadly by the specified warhead size and 330 m/s velocity, but specifically by the availability of surplus fire extinguisher tubes of that size! Unlike the British RPs, the 4.5" (designated the M8) had folding fins as they were launched from tubes. After significant problems with propellant quality, the M8 entered service in December 1943. In service, the tube-launching was found unnecessary and imposed drag penalties, so first the zero-length launcher from the USN's 5" (127 mm) HVAR (high velocity aircraft rocket) was utilised, then the HVAR itself was adopted, although the M8 remained in service alongside. The M8 weighed 18 kg and the warhead carried 2.3 kg of HE and reached 260 m/s. The HVAR, which achieved 390 m/s, weighed 63 kg and carried 3.5 kg of HE."
The Hurri attacked at around 250 mph. If its speed went up to 300, it could have got in three shots. The Tsetse with 6 pdr gun was very fast yet in practice could still get of four shots against a tank-sized target, with a virtually guaranteed hit.
I note that in describing the results in Korea you refer to 'claims'. Please note that WW2 claims against tanks were found to be overstated by 1,000% when the results were examined on the battlefield.
Tony Williams: Military gun and ammunition
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