If it's assur to add and subtract (Devarim 13:1), is the voting system allowed?
The Gemara (AZ 26a-b) says regarding Ovdei Avodah Zarah that lo ma'alin v'lo moridin. Does that mean that you can't vote on Christianity.SE?
Partial answer related to #1, for now.
Voting is not adding or subtracting from "the thing". The beginning of the verse you cited says את כל הדבר - From the entirety of the "thing".
What "thing" are we referring to, here. The Mi Yodeya question, of course. Voting does not add or subtract from the question, itself. It just reflects an opinion about the question.
The bigger problem is if that vote causes the O.P. to edit his question, and thereby, causes him / her to add or subtract from what was originally in the question, itself. Now that's probably a violation of #1. It's possible that by downvoting, exp. or even making comments you might cause the O.P. to make such edits, in which case, you may be encouraging a fellow Jew to sin, which is also a problem.
Nonetheless, voting is not a direct violation of #1.