Your csv file is not here so I do not the details very well but I can try to answer.
1 & 3 - review the online help for Joins - Waterfall Charts - in general the answer is that you end up with one single data source in Tableau. Data blending (q4) is good for summary/detail type views but we have a nice kb on this topic as well - https://kb.tableau.com/articles/howto/deciding-between-joining-tables-and-blending-data
As for the 1/0 question - this is usually due to a calculation utilized for either sorting or filtering. I do not see a calculated field in this example workbook however with a field of this type so I am not able to assist there.
Best of luck