For the gist of your questions, I would have to say that plants are not simple machines or mathematical formulas. There are many interwoven effects that will change everything. When it rains the scent and root exudates will be rinsed from the air and soil. Varying atmospheric pressure, humidity, temperature, or times of plant stress will change all of your variables. The genetic diversity of the plant will obviously change how strong the plant repels enemies too.

Such specific questions can be answered, in a way, by punching them or related questions into a search engine and then extrapolating your answer from what you learn.

I simply typed "marigold" into wikipedia and it was stated that marigolds repel insects via scent and also repel nematodes. For nematode repulsion I can reasonably guess that any area covered by it's roots would contain substance that is undesirable for nematodes (if your raised bed has good soil the roots will go into it). For insect repulsion I can also reasonably guess that if I can smell the flower it would be repelling insects.

The most important thing to take from this though is that nature is variable. You can adjust some parameters to try and sway the system into your favor but there will always be an adaptation to your efforts and something will happen that you didn't expect (marigolds don't repel birds, squirrels, digging dogs, trampling kids, hail, frost, or comets). Just plant extra and have fun with it. :)