Firstly, there are no geographic restrictions on the plaintiff in a US lawsuit, baring forum non conveniens doctrine, which considering you were working in a US jurisdiction and the company was doing business there, should not be an issue.
(EDIT: As user A.Fm. points out, the real first thing to do is check your contract for a clause determining jurisdiction and/or arbitration. If you do, it will determine if and where you can sue them.)
The first thing I would do is to hire a lawyer, preferably one specializing in employment law, and bring/send a copy of your contract. The first step to suing someone is filing a complaint. This is something you will want to have a lawyer do. (Pro tip: Even lawyers generally don't represent themselves in court). However, this is probably not the first step your lawyer will take. They will probably send your former employer a demand letter. It's faster and cheaper for you if they settle than going through a whole lawsuit.
As for your complaints:
- The tax withholding seems reasonable, albeit a little high. You can request your W2 from your former employer, preferably before your lawyer contacts them. If they don't respond, have your lawyer ask them. As a last resort, you can have the IRS send you your W2 for a small fee. If what was withheld from your paycheck matches your W2, you're fine. Otherwise, yes, it appears that they are stealing from you. Talk to your lawyer.
Note that if they are on the up-and-up, they can still over-withhold (the majority of employers do, which is why you hear people talking about their (relatively) big tax return, and what they are going to spend it on); this is not a major problem, just file a tax return (which you are required to do anyway), and your excess withholding is returned to you by the IRS.
The withholding of your final paycheck seems actionable. This is what you should talk to your lawyer about.
This is weird, especially after your employment has ended. Most people get a summary of their W2; they don't fill it out. Are you by any chance confusing the W2 with the W4 (which they should have had you fill that out at the beginning of your work for them...maybe that's why your withholding is so high)? The W2 is their responsibility not yours, although it can cause tax headaches for you (and major legal repercussions for them).