Reading this answer I noticed an embarrassing hole in my basic knowledge. From the answer:
Today is (your birthday, and) also my birthday.
Without the 'also' part we would have
But I think the original sentence (with も) is a different structure. Rather than replacing の with も I think that 私 has been promoted to a topic and も is actually replacing は in this case. Is this correct?
If I'm right about the above then how would you add も whilst still preserving the possessive structure? I feel that 私のも誕生日です and 私もの誕生日です would both be ungrammatical. That would leave me with 私の誕生日もです. But that also looks weird. Finally, employing も's tendency to move to the right in sentences, I would fix this by writing 私の誕生日でもあります. But, if I'm not mistaken, that's a rather stiff and formal sentence.
Are the things I've said above correct? What is the most natural, informal way to express this simple statement? Is it possible to preserve the possessive structure, or must we promote 私 to a topic as in the original sentence?