ok since your question is long i will answer part by part.
[quote:s99wq47w]Do you use the built in functions?
yes, nothing external from C2 until the moment, all can be done inside construct 2
[quote:s99wq47w] Is typewriter effect implemented, or someone wrote a plugin?
you can do that with just one event!
[quote:s99wq47w]How do you handle inventory in your adventure game creation?
Arrays!!
[quote:s99wq47w]For Characters do you import jpg, png walk cycle files or you have to draw all sprites in Construct?
all is done in different graphic softwares and exported into png to Construct2, sounds like you havent use Construct 2 if you ask this kind of question, Construct 2 lets you import any image file and use it the way you want.