Does the snow profile in Fig. 6 correspond to the 30% rule?

Comparing the snow depth under the tree and in the forest opening we would judge the percentage of precipitation not being intercepted by around 50%. However, under the effect of warming sun light snow clumps and melt water falls from the tree crown compacting the snow cover below. Hence, the snow cover under a evergreen tree is not only less deep, it is also more dense than in an opening. (Try to experience it!).

How must a branch look like where snow get caught efficiently and stays for a long time?

The more horizontal surface a branch exposes, the more snow flakes are caught.

Which temperature facilitate that trees keep their snow cover ?

The colder the surrounding air, the less radiation and wind reaches a branch and the more snow stays on the tree. Evergreen conifers in cold winter climates (day length!) may therefore experience extreme loading.