Remember, one question per day.  I'll answer the first here:

We first the tunnel motif on pgs. 10-11 of The Things They Carried.  Lt. Cross' mission is for Alpha Co. to locate and destroy the tunnels in "Than Khe area south of Chu Lai that the Viet Cong used to hide in."  Lee Strunk draws the unlucky number "seventeen" to become the tunnel rat: going down head-first into the tunnels with a gun, flashlight, and rope.

The narrator says that the rest of the platoon were on pins and needles during the tunnels missions: “the waiting" was "worse than the tunnel itself.”

And later:

"...The playing field was laid out in a strict grid, no tunnels or mountains or jungles. You knew the score...There were rules."

Tunnels are symbolic of the unknown.  They represent the harrowing "search-and-destroy" tactics of guerilla warfare.  They are objective corrrelatives for a soldier's fear.  They are psychological devices for exploring the depths of one's inner-most feelings (see Freudian analysis below).  And they foreshadow what will happen to Kiowa, who will forever be buried alive in the "shitfield" later in the novel.

In Freudian terms, Lee Strunk, with the rope tied to his ankle, is like a child tied to his mother (earth) through an umbilical cord.  The caves at the ends of the tunnels are concave symbols that represent the womb, mother-earth, that which all male soldiers desire to return.  Many of the stories and letters in the novel are addressed to women (Cross writes to Martha), and so the women (mother-earth) in the novel represent the comforts and security of home.