NET

Yes, you can appear for NET. The candidate must have completed his post graduation in any stream with an aggregate of 55% without rounding off.

Sorry with backlogs you would not be eligible for SET/NET exam.

The main objective behind writing NET is
1. Elegibilty for Ph.D.
2. Eligibility for a lecturer in a college. Considering the requirement in your case is purely based on a good salary. - which can be achieved if you start working in a private coaching center that offers up 14-15 L initially.

You will have to opt for Life sciences as your subject in the NET exam. Some of the popular books you can refer to are mentioned below:
1) UGC-CSIR NET (JRF & LS) Life Scienceby Ashish Nagesh and Quaisher J. Hossain
2) Joint CSIR UGC NET Life Sciences - Previous Years' Papers (Solved) CSIR UGC NET JRF & LS Exam)by RPH Editorial Board
3) CSIR NET/JRF Life Sciences MCQ Practice bookby Rupendra Singh and Dr Madhu Gupta
4) Life Sciences: Fundamentals and Practice - Iby Pranav Kumar and Usha Mina
5) CSIR-NET General Aptitude - A New Outlookby Christ Varghese.

Yes, you can appear for the NET exam. You need to secure at least 55% marks in your Master's degree or equivalent examination from recognised colleges.

You can refer to the following books:-
1. UGC NET/JRF/SLET General Paper-1 Teaching & Research Aptitude.
2. Truemans UGC NET/SET General Paper I.
3. CBSE UGC NET/SET/JRF Paper 1: Teaching and Research Aptitude.

Yes from July NET will once again be conducted twice a year, and a two-paper exam will replace the earlier three-paper format. As per the new exam pattern, there will be two papers: paper I will comprise objective questions to test the teaching abilities, aptitude and approach of candidates towards engaging students.

You can select the subject you have studied in M.Sc. This subject must be included in the list of subjects given in UGC website. As microbiology is included under the list for CSIR NET you can take the test in the same stream. Please check for detailed eligibility to appear for the NET exam as mentioned at the link provided below:
https://www.shiksha.com/teaching-education/exams/ugc-net.

Economics is the best subject for NET, as you are familiar with economics with your stream and it is also beneficial for your further competitive exams.