Sure you can cover just one branch and flower it to determine the sex. The foil I would be careful it does not retain heat because you could cook the leaves if you are not careful.

When you top the plant it will make the plant grow back bushier. Lower branches will develop quicker and have more flowers. Removing the terminal bud will alter the growth inhibiting hormones (auxins). This is why the side buds will grow quicker with even more foliage and give you even more buds of finished product. Not everyone has room to crop and grow big bushy plants or trees but itā??s worth the extra effort.