It sucks when people that sell stuff don't even know what they're selling.

I do think you can grow weed with just red, which is of course more important than the blue. Many people grow weed with only 3000K fluorescents. Without blues your plants will end up stretching to some degree, and end up less bushy. It's not a problem as long as you are willing to work around it. Your yield will not be quite as high, but again, not a problem.

Instead of mylar I would paint the interior of the wardrobe flat white. It does a better job at reflecting and diffusing light. Also your walls will get dirty and the mylar will soon dull, and splashing another coat of paint on is easier than replacing the mylar.

Looking forward to your tests.