Permanent Residence requirements for spouse of a German

Hello Everyone,

I would like some help clarifying a few points about the Niederlassungserlaubnis. 

I have been living in Germany for 2 years and 6 months, and I am married to a German citizen. I'm not an EU citizen.
From what I understand, I may be eligible to apply after 3 years of residence, correct?

I have a specific question regarding employment:
I have never worked in Germany because I previously lived in Switzerland and since I moved to Germany, I continued working there as a cross-border commuter (Grenzgänger). I do pay taxes and live in Germany, but all my pension contributions are in Switzerland.
How does this affect the requirement for pension contributions or proof of social security payments for the Niederlassungserlaubnis?

Another question:
When I moved from Switzerland to Germany, I first received a Fiktionsbescheinigung, and only about 4 months later I received my residence card, which had a later issue date.
Which date is considered the official start of my residence for the 3-year rule the date on the Fiktionsbescheinigung or the date on the actual residence card?

I already have a German language certificate B1.
Is there anything else I need to prepare, and can I already apply once I reach the 3-year mark or make the request a bit earlier is fine?

Thank you in advance for any guidance.