What is the detailed Process of getting Schengen Visa by locking money for verpflichtungserklärung?

I am from Iran and want to visit my friends in Germany, but getting a visa is difficult.

However, I have a German friend who wants to send me a formal invitation letter called verpflichtungserklärung so that I have a better chance of my visa application being accepted. On the other hand, according to the immigration office of her town, as her salary statement is below €1500, she has to lock €5000 in a bank. She can then access this money when I leave the country which, in my case, is 14 days. She is unsure about the complete process of getting the money back. There is another document called Grenzüberschritts Bescheinigung that she is told to fill, but she doesn't know what exactly it is. To sum up my questions and end her worries, I have some questions and would like to receive your answers. Please provide the source of your replies if you have.

1) What is the exact process of getting verpflichtungserklärung, when her income is less than required €1500 ?

2) How does the immigration office understand that I left Germany, so that they will notify the bank to unlock her money?

3) What will happen if she sends me the verpflichtungserklärung, but I cannot get the Visa? How can she unlock her money and inform the immigration office that I haven't even entered Germany.

4) What is Grenzüberschritts Bescheinigung and when is it needed?

Thanks.