Actually, I'm not sure that this is a typo; however, at the same time, should and shall are not interchangeable in [28iv]. Comparable and interchangeable are not quite the same. They're basically saying that it is being used exactly the same as it is in [28iv].
This has nothing to do with conditionals. It's more a question of semantics. Yes, the text in the paragraph and [28v] are both conditionals (as indicated by if/shall/should and may), whereas [28iv] is not; but, that's not what they're talking about.
In example [28iv], that he should be so late, is semantically the same as that he was so late, just as if the tenant shall fail to is the same as saying if the tenant fails to. There is no change in meaning by removing either should or shall.
And while example [28iv] doesn't work with shall, it's used in the exact same way, meaning that shall is being used in that paragraph in the same way that should is being used in [28iv]. The modal adds absolutely nothing to the meaning. But, at the same time, you can't say it's surprising that he shall have been so late, at least not in Modern English.
That is not the point that the book is trying to make though. They simply mean they're the same usage, not that they're interchangeable. Here's the relevant context: Shall fail here is semantically indistinguishable from fails; this use of shall is comparable to that of should in [28iv].
In other words, that he should be is indistinguishable from was. That's a very literal reading of the text. So strictly speaking, it's correct.
On the other hand, with example [28v], If you should experience any difficultly, please contact me, should also has little meaning. If, though, is completely unnecessary in that sentence. It could be re-written as Should you experience any difficulty, please let me know. Saying if you should experience, is more like saying If you happen to experience any problems, please let me know. Think of if you should as being similar to in case here.
While I'm sure that there are plenty of speakers who wouldn't think twice of omitting should from [28v], it's not technically correct to do so. Although it probably wouldn't change the meaning of the sentence to most speakers if should were omitted, you're asking about a reference book, so it's most likely following prescriptive rules of grammar.
And even if that weren't the case, should adds politeness and formality to the sentence. That's not the case with [28iv].
What I take issue with, however, is that example [28iv] is the one using archaic language (in the US at least). But it's not the same as the archaic American use of shall that is only found in legal documents, which is what they're trying to explain.
This is very poorly explained in my opinion.