05AB1E, 25 bytes

0¸«¥X;D"+(ÿ±ÿ)*("ý¹g<')×J

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Explanation

0¸«                        # prepend a 0 to input list
   ¥                       # calculate delta's
    X;D                    # push 0.5 twice
       "+(ÿ±ÿ)*("          # push this string and interpolate 0.5 where "ÿ" is
                 ý         # merge the list of delta's with this string as a separator
                  ¹g<')×J  # add the closing parenthesis

Might be shorter to buildBuilding the expression from the right.
Need to have a look unfortunately ends up at that the same byte count with
0¸«¥¤s¨RvX;Dy"ÿ+(ÿ±ÿ)*(ÿ). The 8 bytes used for the setup is the big waste here.