that man had to be older than he was, and wearing an emerald-green cloak!
This is a kind of indirect speech (or really thought) reported by the author. The man originally thought "that man has to be older than me", and "that man is wearing an emerald-green cloak!"
The author is reporting these thoughts, and abbreviating them slightly. It could have been more wordy, such as—
that man had to be older than he was, and (that man was) wearing an emerald-green cloak!
As you've discovered, the ordinary order of English declarative sentences is subject first, verb following, but there are a number of rhetorical or informatic reasons to invert that order. One is that speakers tend to put old information before new information, and another is that speakers prefer to place weightier (i.e., more grammatically-complicated) structures at the end of a sentence. It's likely both principles are operating here.
In your sentence
A (adjunct) = Along with the rise of agricultural societies
B (subject) = the development of property ownership and the need to keep records of it
The ordinary order would have
B came A
But my guess is that the paragraphs preceding the sentence had already introduced the topic of the rise of agricultural societies. So that's old information. And the sentence is introducing the new idea of property ownership and record-keeping.
Now, note that A is two prepositional phrases, one using along with; the other, of. Light and simple.
Contrast that with B, which has the structure of a compound noun phrase -- the development ... and the need .... The first noun phrase has a modifying prepositional phrase (of property ownership), and the second noun phrase has an infinitive complement (to keep records), which itself has a modifying prepositional phrase (of it), with a pronoun (it) having an antecedent (ownership) in the first noun phrase. Heavy and complex.
So the principles of old-before-new and light-before-heavy work to invert the ordinary sequence to
A came B.
It is easy to conjure examples of the operation of these principles:
Because of his aforementioned pro-Nazi sympathies, a virtual exile to the Bahamas
for the duration of the war was the Duke of Windsor, formerly King
Edward VIII, a man who had abdicated the British throne to marry an American divorcee.
Without the "aforementioned" signaling an old piece of information and without the "heavy" addition of the appositive, the subject (exile) and the nominative complement (the Duke of Windsor) would more naturally be reversed:
The Duke of Windsor was a virtual exile to the Bahamas for the duration of the war because of his pro-Nazi sympathies.
Best Answer
The sentence means:
Three things make the sentence difficult. The first two are:
This is a conditional sentence, but there is no if. We understand the same meaning as 'if' because in the subordinate clause, the subject the professor, and the auxiliary had have inverted - they have changed places. The usual order would be: the professor had not. Some more examples of this:
The word not can be confusing if you aren't used to this kind of inversion. It may not be clear which word(s) not belongs to. In your sentence, it is part of 'the professor had not'. It has just been separated from had because of the inversion.
Back to the rest of your question. Might shows that it is possible, but not definite, that the lecture would have been boring without some jokes. The speaker is using might, not may, because a past tense modal verb is needed in this type of conditional.
This is the type of conditional speakers use when they want to say what the logical outcome of a situation was (whether the situation was real or not). They usually use it, however, when they know or believe that the situation or event did not happen. We use past perfect verb forms in these conditionals, so we see might have in the sentence instead of just might.
The third thing that makes the sentence difficult is the very long noun phrases. We need to find the head noun in the noun phrase. If we understand the head noun, we will understand the important information in the sentence. In your particular example:
Lastly, not a grammar point, but in case it's helpful: the verb leaven usually means to make a situation feel lighter or less serious.
I hope this is helpful!