can MLV be stacked to provide better results? for instance if one has 1/8" 1lbs per sq/ft can you put two layers on to get double or close to double the effectiveness? if so how should the sheets be orientated? front to front, back to back, or in the same direction? you couldn't glue it together right, because that would lessen its effectiveness?

when installing into doors it should be affixed around the edge but left relatively free floating other than that, right? my understanding is if its rigidly stuck down that it will lessen its ability to absorb sound.

Just to clarify, it is a barrier not an absorber.  Doesn't mean there isn't some overlap or even confusion in that statement, but when you think of it that way it will make things easier to contemplate.  Then your question needs to be is one 1lb per sqft barrier in 2 layers as good as 1 layer of 2lb?  To think of that first start with is 2lb better than 1lb?  That answer is yes absolutely as the only thing doing anything here is mass.  In other words it is not the thickness but the mass.  So yes, double works.  The 2nd part of the question in which is better requires more detail.  Either 2 sheets or 1 doubly heavy could be better based on how it is installed....but that goes in too deep and can be really complicated to predict.

Yes, it should "float"...but a better word is that it should be decoupled.  Goal is to not couple any energy you don't have to into your absorber.  As for orientation, when from above you realize mass is king and add that 100% coverage with no holes is required you should draw the conclusion orientation is interesting only for helping you maximize coverage without strange cutting and such.

Yes, stacking or a adding more dense  MLV (2 lb per sqft and up) has added benefits. And like the poster above said, MLV is not an absorber it is a barrier layer that helps stop sound from transmitting through. We would recommend that you overlap your seams to be more effective, not apply the same size pieces on top of each other.