Useful iNat links
http://www.inaturalist.org/observations/export
http://www.inaturalist.org/observations.csv?day=26&month=8&place_id=49955&year=2013&limit=200&page=1
http://www.inaturalist.org/observations.csv?day=26&month=8&place_id=49955&year=2013&limit=200&page=2
Not any particularly good ways. You can figure out when a taxon was first added to a project by searching for project observations and sorting by "date added," e.g.
That will show you that http://www.inaturalist.org/observations/85957 by everett was the first Gray Tree frog added to the project.
New taxa by month is not currently possible, but you can see all the taxa for a particular month with a little URL hacking, e.g.
http://www.inaturalist.org/observations/taxa?projects[]=vermont-atlas-of-life&on=2014-08