[Laurent Rizzotti]

Thanks for the link, the casualty list is the same as mine and coming from CWGC. As for the 64th Sentai involvement, this part of the text seems unrelated to the rest for me, but the author probably added it from the Osprey book he had. According to Shores book, in May 1942, JAAF had the following fighter and bomber units in Burma:
1 Sentai (Ki-27) - last
8 Sentai (Ki-48)
11 Sentai (Ki-27)
12 Sentai (Ki-21)
14 Sentai (Ki-21)
27 Sentai (Ki-51)
31 Sentai (Ki-30)
64 Sentai (Ki-43)
77 Sentai (Ki-27)
98 Sentai (Ki-21, left mid May 1942)

Last two Japanese raids listed in Bloody Shambles vol 2 are a raid on 5 May against Paoshan (China) by 27 Sentai escorted by 11 and 64 Sentai, and a raid by Ki-21s on 9 May against Chittagong.
By the way I was wrong when I said that Shores described no Japanese raid between 10 and 22 May, there is one: on 21 May Lt Col Kato led 7 Ki-43s of 64 Sentai on a sweep to attack Chittagong airfield but no British aircraft were seen and during the return flight one pilot had to bail out when his fighter suddenly burst into flames and was captured.
It seems to me that 64 Sentai is the Japanese unit covered with the most details by Shores, and so was probably not involved in the raids on Imphal and Silchar.

Japanese air units were based in Magwe, including very probably 77 Sentai who fought air battle against RAF bombers there on 10 and 12 May. 64 Sentai was probably still in Mingaladon, near Rangoon, and moved detachments to Akyab mid-May, scoring first victory on the 17th. 27 Sentai was based in Tongoo in late April, and from there Paoshan and Imphal are roughly the same distance, so any Japanese bomber and fighter unit might have been involved.