I run a perpetual grow scrog - a flower room and a veg room. It takes 60 - 80 days to flower depending on the strain. During that time I'm vegging the next round. I LST the main stem and then top/fim branches as they go. At times I find myself needing to slow the plants down. Topping achieves this, to a degree. Note that while it takes 3 - 5 days for the top/fim to recover, during that time development is directed towards the lower branches. This gets me lots of branches for the scrog screen and hence more bud sites. Topping and Fimming is also a necessity for those with height limitations.

Flat paint is nearly as good as mylar. Note that moisture can get trapped behind the mylar and the wall leading to mold. Similarly, it provides a convenient refuge and breeding ground for pets. Plus at the end of the day when you move you will need to pull it down. Go with the flat white paint and be done with it.