So H1B's effect on long run wages would be better study. The causality of non-normalized reduction in fixed-wage is too chaotic to be attributed to H1b only. Note the year of study. It is 2009-2010, which was height of the depression.
2.Have a structure to it. E.g. Would start at 8 am on Saturday, finish on 8 am on Sunday, upload demos by 10 am, and do anything using X library in python.
Then just let the participants do whatever they want, don't unnecessarily linger or ask too many questions. Remember, people are there to enjoy themselves, and code whatever they think is a good project. Don't judge.