The calculation for me is losing these two "old" BB's vs what ever damage their 14's could have done. warspite1 Subs escorting the battleships? What speed are the battleships going to be travelling to Hawaii? Even if you put the subs to sea ahead of the task force, how many days is it going to take to get them to the take-off point? And then how long for the battleships to travel from there to Hawaii? Besides which subs are hardly a substitute for destroyers when it comes to escort duty. You say they could afford to lose two "old" BB.
No, I didn't say the Japanese could "afford to lose", I wrote that the calculation would be their loss vs additional damage done by them. The possible damage would have to be their 14 inchers and how long they would sit out there pounding away. The time element is critical to the damage vs risk, and of course what sort of damage can the 14's do, and is that damage even worth the risk?
Now if you take what Centuur writes and there is an invasion, then of course the BB's and other support ships would be close in for support.
Concerning the subs: I agree they are not as good as DD's and CA's (best benefit for surface ships probably = AA, anti-sub). The Japanese already had a bunch of subs around Hawaii (Hawaiian waters) prior to the arrival of the fleet:
Squadron 1: I-9, 15, 17, 25.
Squadron 2: I-1, 2, 3, 4, 5, 6, 7.
Squadron 3: I-8, 68, 69, 70, 71, 72, 73, 74, 75. (I-68 and I-69 were stationed off the entrance to Pearl to rescue midget sub crews)
The mother subs that launched the midgets: (I-16, 18, 20, 22, 24)...not sure if they were capable of doing anything more than launching the midgets.
But the Japanese were already paranoid about the missing US carriers. In prior planning they also expected to lose 1+carriers, and the fact they did not was seen as very lucky...that's why the third strike was called off and they headed for home.
Here is a snipet I found that I didn't know about: There actually was an invasion force, but it was intended for Midway. Two destroyers bombarded Midway, Marine shore batteries return fire claiming damage to both ships. Invasion force withdraws (I guess they thought Midway undefended?). Subsequent bad weather saves Midway from a pounding by planes from the Pearl Harbor Attack Force as it returns to Japan. And: 16 Dec. Japanese Pearl Harbor Attack Force detaches carriers Hiryu and Soryu, heavy cruisers Tone and Chikuma, and 2 destroyers to re-inforce the second attack on Wake Island.
Conflict with the unexpected: two qualities are indispensable; first, an intellect which, even in the midst of this obscurity, is not without some traces of inner light which lead to the truth; second, the courage to follow this faint light. KvC