Find a mirror configuration to match laser destinations
Python, 671 654 bytes

Not a solution, but an attempt, read below.

import random as R
def V(F):
 for S,_x,_y in (F[0],0,1),(F[-1],0,-1),([L[0] for L in F],1,0),([L[-1] for L in F],-1,0):
  for i,C in enumerate(S):
   if not C in '+-|':
    x=_x;y=_y
    if not x: X=i;Y=y
    elif not y: Y=i;X=x
    while F[Y][X] != C:
     if F[Y][X]=='\\':x,y=y,x
     if F[Y][X]=='/':a=x+y;x,y=x-a,y-a
     X+=x;Y+=y
     try:
      if F[Y][X] in '+-|':return False
     except:
      return False
 return True
F=open(input()).read().split('\n')
while 1:
 _=[F[0]]+['\n'.join([L[0]+''.join([R.choice(' \\/')for i in range(len(F[0])-2)])+L[-1] for L in F[1:-1]])]+[F[-1]]
 if V(_):
  for l in _: print l
  break


I did not golf this to the max, since I'm not satisfied with the solution.
V validates a given solution by walking the field F for each character C it finds on the sideline. Solutions are generated at random. It's ugly, it works for entry1, but takes a Lot of time for the other entries. Since it randomly tries solutions, I don't consider this to be an actual solution for the given problem; but it might help others.

Run:
echo "entry1.txt" | python script.py