Build it according to the plans. Build more if you need to. You will have more flexibility that way and guaranteed good results.

You have to do everything to prevent any air leaks, anywhere and everywhere.

I doubt that having a "doubled" WO would be desirable, because the drivers would interact with each other instead of a sealed chamber. I have done that and the sound was OK at first, but I didn't try to push it, because just a little over a modest output it got ragged. Try it with a foam gasket between them if you want, but be prepared to close them up and you'll have 2 great boxes.

The second part of your question makes no sense to me, because if you have 12 inches to add to the front for a mouth extension why not put it where it should be and build it according to the plan. I think extending the mouth will have very little effect, unless you get seriously large with it, but increasing the overall internal dimensions back to 36" X 36" will be very beneficial to the low frequency extension.

I am so glad my wicked ones are not an ounce heavier....I would not want to lug around twice the weight.  They are cumbersome and unyielding to say the least. You should try carrying one up stairs.  Maybe some castor wheels on the bottom.... or a fork lift....