partha_pratim wrote:1. I don't have a background in Physics, how do I establish in my SOP or resume, how worthwhile it is for my passion and interest to start a career related to Astrophysics/Cosmology/Astronomy?
This will be very difficult. As you might know, getting into a PhD program and later an astrophysics/cosmology career requires a lot more than just passion and interest. You will have to demonstrate more than this to have a good shot a program.
The majority of other applicants that you are competing against will have 4 years of formal physics education and some research experience. Now, most schools don't have you do 4 full years of only physics courses as most bachelors degrees have breadth too. So you are probably only "behind" on 2 years of formal Physics courses.
So, how can you demonstrate an equivalent level of knowledge and experience? I think taking online courses is certainly one way to get yourself up to speed on the material. If you can take a formal education program, that is even better. You have a few years and it sounds like you have a good job right now to support yourself. So, can you enroll in evening classes at a local college? You don't need a full Physics degree, but if you take the equivalent 3rd and 4th year Physics undergrad classes that would help a lot.
Alternatively, if you can afford to pay for a MS in Physics, then that might be even better. If you enroll in undergrad courses or a MS program, this will also open up potential research opportunities. This will go a really long way towards your application.
Also, a strong Physics GRE score could help show that you know your Physics.
2. Given, I already have an MS, and also 6 years of work experience in the US, do you think schools can waive of the GRE general/TOEFL requirement?
Very unlikely the GRE general will get waived. This is a requirement for everyone, not just international students and it doesn't matter how long you've lived in the USA or how well you know English. It's not a test of your English skill, even English majors take it and people who decide to go to grad school after 50 years of living in the USA have to take it.
Most schools should waive your TOEFL requirement though. Usually the requirement for a waiver is to have completed a degree program at an English speaking University (e.g. your MS degree from Arizona). So, when you are ready to apply, reach out to these schools.
3. I see that the GRE Physics prep book is priced at $500 on Amazon, what is the best way to start refreshing the basics before even looking at the GRE test papers?
I think you can find much cheaper ways to prepare for the test!
That said, note that many astro programs no longer require the Physics GRE. Still, in your case, it might be a good idea to take the test and if it's a good score then you can send it in as additional proof that you know your Physics as well as someone who did a BS in Physics (see #1 above).
4. I could get few professional and academic letters of recommendation, but naturally, they can only talk about my work ethics and judge me as a student and they cant speak about any background in Physics. Will this hold any good?
It is not ideal but given that you have not been in school for some number of years, the committee will take this into account when judging your letters. To give you a honest picture, this means you are at a disadvantage compared to other applicants. That is, schools will look to see if you have demonstrated competency in Physics. For most applicants, there are many sources to show this: transcripts, letters from physics profs, Physics GRE scores etc. For you, it might only be the Physics GRE score. This is fine as long as it's really good but now everything depends on one aspect. But if you do some of the education suggestions in #1 above, this could allow you to get better letters.
5. If I decide to apply in the fall of 2018 (for a program commencing in Fall 2019), I have two years. What could I do in these two years to earn credits (which I could later encash for pre-requisites and get them waived, as I dont have a physics background)?
Note that you don't really have two years. For a program commencing in Fall 2019, the application deadlines are about 12 to 13 months from now. So you really only have one year from today. Some of the steps I suggested in #1 above will take 2 years to complete. It's up to you to decide how you want to spend your time, but you might consider waiting to start in Fall 2020. Or you can apply to a few schools you really are interested in for Fall 2019 and continue working on the steps in #1 while you wait to hear back and reapply for Fall 2020 if you don't get an offer you like.
That said, if you are interested in a Physics MS route, these deadlines are coming up so you might want to act now and start a MS in Fall 2018.