Ok, let me see if I can clear some things up for you. As a J-1, you may be subject to the 2 year home residency requirement. This means that you must leave and be in your country for two years before you can come back to the U.S. You CANNOT marry a U.S. Citizen and adjust status in the U.S. until you have gone home for two years. Had you stayed on the F-1 and not switched to the J-1, then even if you had stayed illegal, you could get married to a U.S. Citizen and adjust status in the U.S. But because you switched to the J-1, IF you are subject to the requirement (which you probably are), then you have to go.
You are in a very bad position because IF you overstay, then you will not be able to get back in for 3 to 10 years unless you have a U.S. Citizen spouse and you are granted an I-601 waiver for any time you spend illegal. These waivers are VERY hard to get. So to clarify, if you stay longer than 6 months illegal but less than 1 year, you cannot come back in for 3 years. If you stay illegal over 1 year, you cannot come back in for 10 years. And because you are in a J-1 that may be subject to the 2 year home residency requirement, then you MUST leave because you won't be able to adjust status in the U.S.
So if I were you, I'd leave for the 2 years and then see if you can come back after that. You could look into applying for a J-1 waiver, but I don't think you have the time. That information could be found here:
If you overstay, yes, you face a risk of deportation. If you overstay less than 6 months, there is not automatic bar for getting another visa and coming back in, but they will hold that against you when they use their discretion in deciding whether to grant you one or no, so it's best not to overstay. If you overstay longer than 6 months but less than 1 year, you activate the automatic 3 year bar to re-entry. If you overstay longer than 1 year, then you activate the 10 year bar.
If you leave legally, you should not have a problem coming back in as long as the immigration officer doen not believe that you are living in the U.S. (meaning that you are staying more time here than in Germany) and to help prove this, you should carry proof with you that you have a job in Germany, relatives, property, etc. etc.
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