There is a hole in your explanation.

You say: 

"In sentences where the indirect object is represented by "a + pronoun", and it is at the beginning of the sentence...."

but you do not explain what happens if "a + pronoun" is at the end of the sentence.

Is the indirect object pronoun required or optional in that case?

eg: Can "Me lo ha comprado a mí." be written as "lo ha comprado a mí."?

Regards,

Fred.