The Mechaber 607:3) paskens that if you are present when the shat"z says viduy you have to stand. Rama adds that you have to recite viduy with the shat"z. The implication is that the Mechaber holds that even if even if you are not saying anything along with the shatz you still have to stand (which is how the mishna berura explains it). Why should that be true? If you are not saying viduy, why do you have to stand?
I don't think it is a din in shome'a k'oneh, and nothing to do with any chovas hagavra to say viduy or stand while saying it. It's a seperate din in the definition of viduy -- the cheftza of viduy is something recited in the context of a tzibur which is standing.
Speaking of shome'a k'oneh and viduy, the Rosh in the last perek of Yoma (#25) quotes the view of Rav Amram Gaon that on Erev Y"K the shat"z should recite viduy out loud in chazaras hashat"z to be motzi someone who is aino baki and does not know how to say viduy him/herself. The Rosh and most opinions disagree. I would guess that shome'a k'oneh does not work to be motzio somepne in viduy because although the ma'aseh mitzvah of viduy is a recitation of sins, the kiyum mitzvah is only effective if there is a corresponding sense of regret in one's heart. Shome'a k'oneh does not work where there is a ma'aseh mitzvah but no kiyum. For example, one cannot rely on shome'a k'oneh for the mitzvah of kri'as shema (see Shnos Eliyahu of the GR"A on the first Mishna in Brachos) because the recitation alone is not the full mitzvah; it must be accompanied for a kabalos ol malchus shamayim which is done b'lev.