#JavaScript (ES6), 258

A function with the maze as a parameter, returning the output.

Unsure if it's valid, due to the input/output rules (it was fun anyway)

<!-- begin snippet: js hide: false -->

<!-- language: lang-js -->

 f=m=>([...m].map(c=>{if(c<' ')x=sx-=2,y=sy+=2;else for(x+=2,y+=2,d=c>'0',i=y-3*d,j=x-3*!d,c=5;c--;)o[i+=d][j+=!d]='#';},w=m.search`
 `,h=m.match(/\n/g).length,sy=y=0,sx=x=h*2,o=Array(z=(w+h+1)*2).fill(' ').map(x=>Array(z).fill(x))),o.map(r=>r.join``).join`
 `)

 // LESS GOLFED

 U=m=>(
 w=m.search`\n`,
 h=m.match(/\n/g).length,
 o=Array(z=(w+h+1)*2).fill(' ').map(x=>Array(z).fill(x)),
 sy=y=0,
 sx=x=h*2,
 [...m].forEach(c=>{
 if(c<' ')x=sx-=2,y=sy+=2
 else for(x+=2,y+=2,d=c>'0',i=y-3*d,j=x-3*!d,c=5;c--;)o[i+=d][j+=!d]='#';
 }),
 o.map(r=>r.join``).join`\n` 
 )

 // TEST
 out=x=>O.innerHTML+=x+'\n'

 test=`\\\\/\\\\\\//\\/\\////\\\\/\\/
 \\/\\///\\\\///////\\//\\/
 /\\\\\\//\\//\\////\\\\//\\\\
 \\////\\//\\//\\/\\\\\\\\\\\\/
 /\\/\\\\///\\\\\\\\/\\\\\\\\/\\\\
 \\/\\//\\\\\\\\\\\\//\\/\\////
 /\\//\\\\///\\/\\///\\////
 \\/\\\\\\//\\\\/\\\\\\//\\\\/\\/
 //////\\\\/\\\\/\\/\\/\\///
 \\\\/\\/\\\\////\\/\\/\\\\/\\/`
 out(test),out(f(test))

<!-- language: lang-html -->

 <pre id=O></pre>

<!-- end snippet -->