1) Any write-up you've done of your work (whether it be a journal article or a thesis, or a paper for school) does say something about your future abilities as a scientist, and along that vein, an admission committee may be interested in it. It says that you did more than wash dishes for two summers. Would I bring it along to an interview? Maybe to have just in case an interviewer was interested, but that's just me. When you communicate with programs during the course of the next few months, you may at some point choose to send it to them (ask them if they'd want it !).
2) If you contributed to the work that went into publishable manuscripts from your lab, why was your name not on the pub? Like I said before, anything that A) is evidence that you contributed on an intellectual level to the research and B) demonstrates your understanding of your work and the field will be of interest to the admissions committees (some things more than others, of course). So these manuscripts from your lab will definitely have some weight, but it needs to be clear what role you had in their genesis.
3) Not sure about the AMCAS "rules" on this one with school credit and what not...but it is good (maybe elsewhere, like your personal statements/secondaries/MDPhD essays) to note that you did present your work (the implication and important thing here is that you presented to people who were not familiar with the specifics of your work, including potentially defending it, which is an important skill).
Good luck!