Go to opposite corner of rectangle in all directions - Harder

Pyth, 46 45 44 bytes

eSKtMQjsM_mm?sJ,kd?q#J.umtWbbNeSKK\#\.\XhQeQ

Try it here.

Explanation:

move-count-printing:

  K     assign K to...
     Q  the input, a 2-length array...
   tM   with each element decremented
eS      take the max and output it (this is the number of moves)


main path-finding logic:

    mm                     hQeQ  map over x-values and y-values...
        J,kd                     assign J to [x,y]
      ?s                         if both x and y are NOT zero (sum is truthy)...
            ?q#J[...]              if [x,y] is present in [...] (see below)...
                     \#            ... put a # at this position in the output
                       \.          ... else, put a . at this position
                         \X      ... else, put the X here (at [0,0])
jsM_                             reverse and output in the proper format


the [...] part in the code above, which finds positions where #s go:

.u      eSKK  cumulative reduce on <number of moves> elements, starting at K,
                which is [max_x, max_y] as assigned at the beginning
  m    N      map over x and y...
   tWbb       decrement, only if the value is > 0

JavaScript (ES6), 132

Edit 2 bytes saved thx @Neil

(w,h)=>[...Array(--h)].map((_,i)=>R(w-i)+'#'+R(i),--w,R=(n,c='.')=>c.repeat(n>w?w:n>0&&n)).join`
`+`
x${R(w-h,'#')+R(h)}
`+(h>w?h:w)

Test

f=(w,h)=>[...Array(--h)].map((_,i)=>R(w-i)+'#'+R(i),--w,R=(n,c='.')=>c.repeat(n>w?w:n>0&&n)).join`
`+`
x${R(w-h,'#')+R(h)}
`+(h>w?h:w)

function test() {
  var w,h
  [w,h]=I.value.match(/\d+/g)
  O.textContent=f(w,h)
}  

test()

Test <input id=I value="4 5"><button onclick="test()">-></button>
<pre id=O></pre>