Can I go back to the US after only six months?

You can return to the US at any time if you have a valid visa within six months or even less after a previous entry. But remember that a visa is only an application for permission to enter the US and it may be denied. You should be ready to provide explanations why you need to come to the US again after a previous visit, or else, the immigration officer might suspect you are not a legitimate visitor

There is technically no required length of time you must remain outside of the country on a multiple entry tourist visa, however, the length of time and regularity of visits could give an office at the port of entry an indication that you have intent issues- meaning you intend to stay in the United States permanently. So, if you're planning to visit friends, be prepared to establish your purpose for your visit. If you wish to stay long-term (several months) you'll want to be able to establish reasoning for that, which makes it clear you're still planning to return to you home abroad. For example, if your employer allows you to take 4 months of vacation, which you plan to spend in the United States exploring the country and visiting with friends, be prepared to establish that- show your travel itinerary, hotels booked, approval for vacation from your employer, etc. At the end of the day, it will come down to whether or not the officer believes you're a true tourist or someone seeking to establish a home in the United States.

It really depends on how long you stayed the last time and how long you have been back in your home country. You cannot be in the US for longer than you are at home in your country because then it does not look like you are just visiting the US but rather living in the US and visiting your home country. That is not permissible as a tourist.

If your B-2 visa is still valid, it will depend on the Immigration officer at the point of entry.