Per the University of Ohio Extension, yes, it does reduce the risk (emphasis added):
Flavored oils also can be a concern if not prepared correctly. When
herbs, garlic, or tomatoes are placed in oils, the botulism spores on
the plant material can start to produce the toxin in this anaerobic
mixture. To be safe, keep these flavored oils refrigerated and make
only the amount of herbal oils and butters that will be used in a
few days. Using dried herbs and vegetables will also reduce the risk.
Still, reduce is not eliminate. The guidelines to not can or hold the oil for long term continue to apply.
Are there any systematical studies available?
I am not aware of any such study or resource, but ignorance of or lack of evidence of a thing doesn't necessarily mean that it doesn't exist.
Which spices can easily bear long durations of broiling, hot oven or barbecue?
Broiling is an especially intense cooking method, directly transferring energy to the surface of the target food by radiation. It also is not a method used for long cooking duration, because it is so rapid, and easily would burn or char the outside of the food.
As a general principle, hard, woody spices and hard seeds tend to be the most heat insensitive: black pepper, cinnamon, anise, nutmeg, cumin and so on.
In the herb family, the hardier herbs (bay leaf, oregano, and sage for example) can stand up to some prolonged cooking without adversely affecting their flavor.
Delicate herbs (cilantro, parsley, tarragon, basil for example) do not fare well as their flavors are very volatile.
Which lose flavor or deteriorate quickly?
See above; this is just a rephrasing of the same question.
Which develop displeasing flavors?
This is an open ended list question.
For a significant portion of the population, cilantro starts with a displeasing flavor.
Badly overcooked garlic can become bitter, if you consider it a spice. Similarly, paprika and many of the capsicum peppers become very unpleasantly bitter when burned.
Most flavors simply lose their intensity and become muted or difficult to perceive.
Which become unhealthy or even poisonous?
I am not aware of any.
What can be done to preserve sensitive spices during hot cooking?
Don't add them during prolonged cooking; add them at near the end of the cooking period or even after cooking is completed.
Best Answer
Depends on the oil, when we infuse like to think of it as steeping not boiling or heating to the point of just below the smoke point as the spices will cook and not infuse, some will also become bitter. I also suggest you put your chilis in all at once or toward the end of the infusion process if you are layering the flavor or want a less intense flavor. Also are your oils for dressings or for cooking? If they are for dressings they will keep their integrity when your process is finished, heating or cooking the oil will alter the taste you may be aiming for. Two different animals. Lastly I personally prefer grape seed or light olive oil for infusing.
Cheers!
EDG