Punctuate:

My orchard was often robbed by schoolboys and my wife's custards plundered by cats.
I think the answer would be:
My orchard was often robbed by schoolboys; and my wife's custards, plundered by cats. Am I correct? Why/ Why not?

Incorrect.

Why did you put a semicolon after "schoolboys"? Why is the word "and" in there?

The whole sentence seems very awkward to me. Do you think some words may be missing?

Yes, you are correct. The correct punctuation for the sentence would be:

"My orchard was often robbed by schoolboys, and my wife's custards plundered by cats."

In this case, a comma is used before the coordinating conjunction "and" to separate the two independent clauses ("My orchard was often robbed by schoolboys" and "my wife's custards plundered by cats"). Additionally, a comma is used before the noun phrase "my wife's custards" to set it off as a nonrestrictive element (giving additional information about the subject "my wife's custards").

Yes, you are correct with your punctuation. Here's why:

In the original sentence, there are two separate clauses: "My orchard was often robbed by schoolboys" and "my wife's custards plundered by cats." These clauses can stand alone as separate sentences because they have their own subjects ("my orchard" and "my wife's custards") and verbs ("was often robbed" and "plundered").

To punctuate the sentence correctly, you would use a semicolon (;) to create a stronger separation between the two clauses. This is appropriate since they are closely related and reinforcing each other. Additionally, you would use a comma (,) before the conjunction "and" to introduce the second clause. The comma after "custards" in your revised version is also correct because it separates the clause from the additional information about the custards being plundered by cats.

So, the revised punctuation would be: "My orchard was often robbed by schoolboys; and my wife's custards, plundered by cats."