The difference between will and shall is the stuff of editorial legend - part nuance, part fading mood, and part shibboleth.
1) Nuance. Like may and can, shall and will convey subtly different messages that the average writer will leave to the pundit to debate. But those whose writing contains legal or regulatory implications should pay careful attention to the difference.
Will implies that something is going to happen. Shall implies that someone is to ensure that the events take place. In other words, shall imparts responsibility; will is merely descriptive. Consider the difference:
The refuge will continue to act as a watershed for the foreseeable future.
The refuge shall continue to act as a watershed for the foreseeable future.
The first describes a natural state of affairs. The second implies that someone will work to create watershed conditions.
Should we fail to make the 10:30 flight, we would also have to cancel our dinner reservations.
In this respect, the verb can be substituted for the less complicated if, which also expresses a conditional statement. Compare:
If we fail to make the 10:30 flight, we would also have to cancel our dinner reservations.
In writing, it is perhaps preferable to use the latter, since it’s less jarring at the start of a sentence. Still, the decline of the subjunctive in English offers a case for the use of should. It makes it clear that would is the appropriate corollary. Otherwise, a writer might be tempted to say we will also have to cancel our dinner reservations – a statement in the future rather than the conditional. So the use of should clarifies and highlights the conditional, and that is to my mind at least a step down the right grammatical path.
3) Shibboleth. Finally, in England and retained to this day among the hyper-literate remains an awkward and unnecessary preference for shall and should for neither of the above reasons.
Historically, those terms were so customary in England for either the plain future or the plain conditional that by the early 20th century, the Oxford English Dictionary described the use of will or would as “a mark of Scottish, Irish, provincial, or extra-British idiom.” (Obviously, that would include American as well.)
During the 20th century speakers responded to this criticism, such that many Americans began overdoing the use of shall and should in an attempt to sound more British; hence, the shibboleth.
Whose English is the right English – the English of the English, or the English of those whom England has spawned, particularly when those whom it has spawned attempt, sometimes poorly, to imitate their mother tongue? In other words, as Fowler frames it in his Modern English Usage, which is better, shibboleth or sibboleth?
The question seems to me a moot point. In most cases, should and would in the plain future or the plain conditional are interchangeable. That’s why this usage is described as idiomatic. As you’ll recall, idioms are conventional usages of language with little or no logical basis.
So long as we employ rule #1 in government and regulatory writing, and so long as we are careful to distinguish between the future and the conditional, I say let us mix our shoulds and woulds as we will (or shall).