ORIGINAL: Lokasenna
I would like to point out, again, that the only difference between B6N2 and B6N2a is radar. And the B6N2a's radar will not activate until 11/44 regardless of the arrival date of the B6N2a, if my understanding of devices is correct (you can't speed them up). So while it might be worthwhile to research the 2a so that it arrives prior to 11/44 and you have a pool of them built up, there is otherwise no real benefit to researching it earlier than 9/44 or 10/44.
I think it's fine to use "corners of the pools" airframes for kamis. They aren't ideal, but if you're going to have the airframes sitting around anyway... Look at it this way: you've already spent the HI to produce those planes. May as well get some return on that investment, even if it's just eating up CAP ops points, right?
Also, the Oscar-IVs and such with their decent speed and 2x250kg bombs are still good kamis. Nates, on the other hand...if you're using them as kamis you're really scraping the bottom of the barrel, but you're probably in 1946 by then, right? So you won anyway!
I have to disagree about B6N2a. It is worth reserching.
On pure logistical side it goes like this; Presuming, that B6N2a arrives around January 1944 (12/43-3/44) and you upgrade your TB production (realistically around 30-50 planes/mo) immediately to new model and start to upgrading your flying units right away (around 25-30 squadrons, averaging something like 15 A/C per unit = around 400 aircraft) you will be close to done upgrading your units when the radar goes active. All this is free of any supply/HI costs and provided that you built a normal combat loss pool of B6N2 model, you never run short of planes. Thus, when 9/44 turns into 10/44, every frontline squardon is suddenly radar equipped.
Also B6N2a has a heavy defensive machinegun, versus 7.7 mm on B6N2 model = higher gun rating.
Drawback is that you keep few R&D factories tied up, but you will have several others free around same time, so this is not something Japanese needs to worry about, in my opinion.
About Kamis... My personal opinion is that fighters should be used as escort, not kamis when possible. Even if flying with completely green pilots, in obselete crates that are hacked from the sky by Hellcats and Corsairs at will, it is still worth doing as they tie the CAP up for those few minutes they take to die. Buying time and reducing CAP numbers (at least few will run low with ammo) while the kamis with decent bomb load (medium bombers, D4Y4s, Tobedo bombers, Ki-115s) head towards their targets.
Also, I got the idea somewhere that plane's durability affected the Kami damage? Was I wrong?
Edit;typos.