Hi, Welcome to the site!
Do not get discouraged so soon. Although these repairs are labor intensive you can get it done, and LOL, you have come to the right place for assistance.
Ok, we will start at the beginning, 2 layers of wet plywood and 1 layer of particle board, so in essence you are saying that there is/was 3 layers of flooring.
The sub floor is one layer. First your have floor joist, which is covered with OSB, particle board or plywood, but no more than one layer. That is your sub floor, you are not suppose to have multiple layers. Since you do, I have to assume that someones fix was to lay all this other wood on top and failed to fix the plumbing leak!
OK, you have removed everything..let your floor joist dry out. If need be put a fan in the area and get the air really moving. Remove some skirting if able to and get even more air under there.
If the joist are showing signs of rot, you should replace them, or at the very least sister them with 2x6's.
Remove all damaged insulation.
Underbelly is moldy? Are you meaning that the belly material has mold on it? If so, spray with a bleach solution, and make sure this area dries real well.
A vapor barrier is a sheet of plastic at least 6 mils thick, that goes from end to end side to side of your home. Overlay all edges by at least 6 inches and seal with tuck tape. Bring all the edged tight to the skirting and the piers that your home sits on. The plastic barrier keeps evaporating ground moisture from penetrating the bottom of your home. Some folks do not use this but in some areas such as mine it is code. Check with your codes office to see if it is mandated. If your home is naturally damp underneath I would put it done if nothing more than for preventive maintenance.
Now, when you replace your flooring remember that you only need one layer of wood, preferably plywood and then cover with full vinyl in the bathroom/laundry area is the most recommended way.
I will leave you with a link that shows you how mobile homes are constructed.
Since you are good with your hands and sounds like you will be doing your own repairs then I highly suggest you purchase The Mobile Home Repair and Upgrade Manuel from this site. It can be located in the BOOKS/PARTS link above. This Manuel has great pics and easy to follow instructions.
If we can help further please keep asking your questions.
Have a great day...
Oh yeah, Congratulations on your marriage!
oops here's your link http://www.mygreathome.com/fix-it_guide/diagram.htm