Is it legal for a mortgage company to show up at your home when you are behind a payment? And to also demand a post dated payment?

Absolutely not.   This is debt harassment.  You can make them stop by sending cease and desist letter.    You can also sue them for debt harassment.   All of this is outlined on my website:   www.consumerrightsorlando.com   If you have any other questions, you may contact me at njtlaw@gmail.com