Butchie,
Your cabinet is very similar to mine. My walls are flat white in fact everything is white so putting mylar will help but not significantly, as you saw from my grow white works fine, Also I got my plants to grow so many collas by doing extensive LST. They were never more than 8" tall for 5 weeks.
Also I use MG moisture control I saw you used that in the past and had good luck with it, now you are having difficulty with the new soil mix, so go back to what you know works if you can.

Then for starting cloans I recomed using rockwool cubes, they hold water a lot better and are much neater. Then for a carbon filter I use 2 - 4'' can filters
Have you seen HTGsupply.com it is one stop shopping for your needs at great prices.

Hope this helps you.