Hi. Thanks for asking this. It's a good question.
Since you are an indian resident, any taxable income of yours is taxable in India.
A1. Since 44ADA is a presumptive taxation section, you can keep using the section till your gross receipts are upto 50 Lakhs.
As per the provisions of section 44AA(1), any person carrying on a specific profession should maintain books of accounts. As you mention that you have business, you would be covered under section 44AA(2), which states that if the income or turnover of an individual exceeds INR 2.5 Lakhs or 25 Lakhs respectively, the assessee must maintain books.
And then, a person carrying in business can be assessed under presumptive taxation u/s 44AD.
Now if you provide the exact nature of your work, it will be helpful in identifying which section you are covered under, whether 44AD or 44ADA.
Accordingly, we will go ahead with the counselling.
Anyway, assuming you are covered u/s 44ADA,
A1. As per section 44ADA, one needs to maintain books u/s 44AA and get them audited u/s 44AB if one prefers to disclose a lesser income than the mandatory 50% under this presumptive taxation. However, if he does not want to disclose a lesser income, then he can simply declare 50% and then, it will be assumed that he has been provided with all the exemptions and deductions u/s 30 to 38 and 28 to 43C wont be given effect. In this case, even though maintenance of books is still required u/s 44AA(1), Audit is not compulsory.
A2. Since 44ADA is applicable to all assessees carrying on profession referred to u/s 44AA(1), with gross receipts upto 50 Lakhs, 44ADA can be applied.
A3. Yes, you are bound to get registered u/s 22(1) of CGST A/c.. edit...But if you are saying that it's US LLC, it gets a non resident status and accordingly, the provisions applicable to a non resident will be applicable. As per section 5, any income of a non resident arising or accruing in India is taxable in India.
Pl seek clarification if required.
Kindly also share feedback. Thanks