Here is some advice for your questions

1.  The expansion gaps are left for a reason.  Your  floor will not move around unless you are having long jump competitions on the floor.  the weight of the floor itself plus the furniture on it should limit noticeable movement.  If the floor cannot move as a unit over time it will work itself apart creating gaps.

2.  Vents are easy but for do it yourselfers time consuming.  U need a table saw, jig saw, and I prefer to use a sliding compound miter saw.  Put the vent in the vent hole and trace the outside edge of the entire vent onto the subloor.  make your cuts 1/2" inside the traced edge so you know your vent will cover the edge of the laminate.  I try to make the fit of the laminate and vent tight as possible so the vent cannot slide around.

3.  As for your crawl space you need to get a professional installer for that  if you want to make it look nice.  Endcaps can be used along the edges of the hole itself and the crawlspace door itself.  I just recomment using carpet in the master closet.....usually where the crawl space is located.

Sorry I don't have an answer to your question since I'm new at this stuff but wondering if you might have some ideas for me.  

I bought the Royal Cherry Hamonics at Costco.   The condo homeowners board of directors seem to feel the Harmonics underlayment is not good enough.  They feel I need something like cork or something else that will provide an excellent sound barrier.  I'm on the 3rd floor and I want to be thoughtful of my neighbor who lives on 2nd floor.