Hi All,
I have lived and worked in Frankfurt since 2012, I was initially on  a deputation here and started with a regular residence work permit (under section 28/29 - not sure) which lasted until 2015. Since then (more than 2 years) I have a local contract and been on a Blue card, that is when I also started paying my SS contributions.
I needed to renew my permit, since the original BC was only valid for 2 years (due to the first contract I had being a limited one). When I went for my appointment for renewal in Feb (around 23 months BC validity), I applied for a Niederlassungserlaubnis since I was eligible for it under the 21 months' rule of the Blue card.
It has been 8 weeks, but I am still awaiting the card/letter to arrive. On enquiring I was told they do not know how long it will take. I have a Fiktionsbescheinigung valid until end of May, and they said If I do not get it until then (almost 3 months later) I should go to them for an extension (and I cannot get an appointment, I must go stand in the queue). The FB states that my employment is limited to my current employer, since the officer told me that they have to put the conditions of the original BC zusatzblatt on it.

Now, I have an offer from a company that I want to take up, but have the following questions:

1. Am I obliged to inform the ABH before I resign from my current job.
2. If I do not receive the NE before I join the next job, can I get a different interim permit other than the FB, which states I can work freely (as per the 24 month BC rule)?
3. Is there a stipulated timeframe within which I should be informed of the NE decision?

Apologies for the longish background, but I thought it is relevant. I would appreciate any response on these queries. I must resign before the end of the month if I want to be able to make the deadline of joining the new job, so there is also some time pressure for me to figure this out. Will be grateful for any guidance. 

Thanks in advance!!
MR