Whether it be/is

"be" is more formal than "is".

In (a), I prefer "... that they left late ...".

Of the options given, I would in conversation most naturally say "... whether it's because ...", but I'd probably repeat "because they" after "or".

I agree with MrWordy, but in casual conversation I'd probably cheat and repeat only the "they," skipping the "because." I'm sure the "because" would be "understood" in my mind.

The reason I prefer (b) over (a) is that the word "because" renders the word "reason" superfluous.

Also, there are occasions when I feel old, and at those times I enjoy using the subjunctive (be).

whether the reason be No.

whether it be that ... or .... OK.
whether it be because ... or (because) .... OK, but not typical.

My impression is that "whether it be" is a more-or-less fixed expression typically followed by very short descriptive phrases, rarely full clauses. Besides, I find it literary more than conversational.

whether

Hi, Jim.

Do you accept "Whether the reason is. . . "?

Best wishes, - A.

Yes, of course. I'm just saying that in modern English, about the only case that survives of whether ... be is whether it be, though an occasional whether he/she/they/I/ ... be might slip through!

CalifJimYes, of course. I'm just saying that in modern English, about the only case that survives of whether ... be is whether it be, though an occasional whether he/she/they/I/ ... be might slip through!

We differ a little here. In British English -- in my experience anyway -- whether he/she/they/I/ ... be survive fairly ha

Be he alive or be he dead
I'll grind his bones to make my bread.

Mr WordyWe differ a little here.

I suspect the objective difference is nil -- merely an artifact of our placing an arbitrary dividing line in a slightly different position.

CJ

AvangiBe he alive or be he dead

I notice there is an inversion when 'whether' is omitted.

My grandfather used these old expressions, and he was a working man. He didn't say much, but he was clearly understood.