If you are already doing MBA then why do you want to go for an US MBA unless you want to go to top 10 Bschool in US.
1. 600 score is decent enough for a tier 2 school but honestly speaking it will not add much value if you are already having a degree.
2. You can give GMATprep test1 and 2 and evaluate where you stand.
3. The reco letter from your professional boss is just a reco letter. What count for adcom is your work experience and career goal.
4.For verbal I would suggest go for MGMAT SC
and Powerscore CR
bible. They are unbeatable sources for fundamental building
Nobody dies a virgin, life screws us all.