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He can travel alone later, as long as it is within the 6 months (validation time of the visa). This is if you wrote that he would be travelling with you.
If you wrote "To join" he will not get a visa at the same time as you and will have to apply again to get the visa, but on the positive side, he would get more time to finish school than the 6 months.
However, if one of your children is reaching the age of 21 years he will have to travel before his birthday and apply for an adjustment of status before his birthday once in the US.

He can travel alone later, as long as it is within the 6 months (validation time of the visa). This is if you wrote that he would be travelling with you.
If you wrote "To join" he will not get a visa at the same time as you and will have to apply again to get the visa, but on the positive side, he would get more time to finish school than the 6 months.
However, if one of your children is reaching the age of 21 years he will have to travel before his birthday and apply for an adjustment of status before his birthday once in the US.

You have to provide your tax transcript for the year before you apply. From what I understand you won't be able to produce it with qualifying level of earnings. Finding a job before the interview to show pay stubs will not meet that requirement. Your only recourse will be a co-sponsor but he will have to produce all the necessary documents. On the other hand, if you can prove that you have enough money to retire, you may be able to get away with it. However, I would also question how you are going to support your fiancee in US if you cannot find a job after 3 years. Maybe you should rethink the whole thing. My VN girlfriend has been waiting for the last 10 years without any problem as life in VN is not bad at all. We talk everyday and I visit 1, 2 or 3 times a year. There will be a big drawback for the future filing if they deny your case once.