I'm not sure how "Of course, I should, and I will." translates to the "absolute certainly" you accord my statement.
Well, "I will" means "I will," not "I will try to the best of my abilities" or whatever ...
Fowler seems to agree with justajane's position here. See
The King's English (2nd ed 1908),
ch 2. Fowler uses very complicated prose, and modern readers might help the original difficult to follow, so I summarise it below.
According to Fowler, "I will" means "It is my will..." and it has two possible uses in English: (1) the "pure" usage, in which case it merely expresses the speaker's wish and does not make any prediction about the future (his example is "I will have my way."), and (2) the "coloured future" usage, where "I will" is used in conjunction with another verb to make a qualified statement about the future. The difference between the two usages is that the first is a statement about the
present and the second is a statement about the
future. From the context, it looks like justajane intended the second use. The reason that Fowler calls this the "coloured future" is that, although it is a statement about the future, it is "coloured" by the fact that it might not come to pass and is merely a statement of
intention, not a definitive prediction (which is consistent with what justajane said she meant).
The original post by justajane also said "I should", and Fowler analyses that as giving a "conditional command" to oneself. In other words, in saying "I should", justajane is saying that if she is in fact on a jury in the future, she will command herself to act as she described in the post.
According to Fowler, the correct form of the uncoloured "plain" first-person future (an "impersonal prophecy" as Fowler calls it) is "I shall...", which justajane did not say.