I'm not exactly sure what you mean by the first question? But I'll try to explain:
With guard, many performers (especially at the world class level) also participate in winter guard, where they are receiving instruction and training during the drum corps "off season." The performers may also keep up their skills by teaching with local high schools or even marching their college programs. Many guard programs (both corps and winter guard) also encourage performers to take dance classes.
In a field show, the guard, being a large part of the visual aspect of the corps, should have sufficient interaction with the drill writer to state "this is where a big visual impact should happen. This is where my transition to new piece of equipment should take place. Here the focus should be on the rifles, not the flags, here the focus should be on the duet dancers, then take them out of focus during this part... etc."
Of course, at the upper levels, you have to trust that the drill designer is considering many of those aspects of the guard integration and responsibilities, and provide feedback based on what is produced. Also, there are designers who will start by writing work and then trying to put it into the drill without having seen the drill, and then tweak the work as needed.