First & Foremost pl ensure you have ALL your paperwork in place--you may not get a chance to do so once you go out into the open with this issue. Once you take this issue into the open, only the paperwork will be what others will ask....NOT what you have to say.
I feel mailing to NASSCOM & the local Indian High Commission will be the right step--but suggest involve some lawyer too--since there could be legalities involved as you go along.
Also, suggest wait for others to respond...there could be better ways of handling this issue.
By paperwork, I mean your Appointment Letter, other company-related documentation w.r.t. your role, performance, increments, promotions, appraisals, etc--whatever could be useful--DON'T ASSUME, just go by what you FEEL, NOT think.
Also included in this docs list should be ALL the mails/correspondence related to your present onsite position, the issues/problems that you are currently facing, etc. If you FEEL something could be useful, but you realise you don't have that aspect/point in writing, suggest put it in writing NOW--pl note it has be very subtle & not give away what you are trying to do.
Pl note that a smart HR guy will see thur the game if you aren't very discrete--this is where the lawyer comes in [to guide you what COULD be useful legally, IF it reaches that stage], to build-up the written part of the case.
With reference to Mr Taj Sateesh's Advise, maintain all documents in your possession and provide copies of the documents to any concerned parties,
Any agreement or SLA signed before going onsite
Copy of the Policy regarding onsite deployment.
Bank Statements reflecting your salary deposits
You may Approach Indian Embassy and explain your case to consular officer and seek advise. You may write an application explaining your case very clearly and with facts including dates and time of events as much as possible.
Maintain strong communication by mails or in writing with your current organization.
Speak to HR department and seek to resolve this issue amicably, if not then approach authorities to resolve your problem, keep your manager involved in all your communication.
Maintain expense report of your basic spending and expenses on behalf of the company. clarify extent of reimbursements you can claim from the company and procedure for the same.
Request for break up of salary and allowances you receive in compensation from HR or Accounts Department.
Meanwhile keep doing your work with sincerity, don't give any chance to company to prove you wrong such as disobedience, not productive, etc etc.
Maintain good name of the company to clients and others etc, keep your calm and work through the problem. come out win win for you and company.
Wish you all the best.
Frankly, it's tough to answer your question with a degree of Finality: what will be the impact on my future career?
The reason being, there are many grey zones in the PoA that are NOT in your control, except responding as you go along. Sort of: developing a 'let's cross the bridge when it comes' mentality. That way, you keep ALL your options open [to the extent you can, i.e.] & also pre-empt any unforeseen surprises to effect your responses.
But coming to your other query: how will I prepare for it--this is what I usually follow myself: Work for the Best-case-scenario & Prepare mentally for the Worst-case-scenario.
This is the General way of handling the issues where one doesn't have a lot of control on the outcome.
Coming to your specific situation, First & foremost, pl try to handle things SMOOTHLY. Just bear in mind that even the PoA of approaching the Indian High Commission and/or NASSCOM is only to bring about the necessary pressure
on your Company--since most companies act only when they realize that their deeds/mis-deeds are coming into public domain. Pl be clear in your mind that HC/NASSCOM will NOT solve your problem--they can only AID your own efforts. The other reason why approaching them will help you [in the long run, if not for the immediate problem's resolution] is that the Company will hesitate to put you into blacklist--which could mar your career.
I hope I have answered your queries.