According to this, on the Torah level a person is not obligated to be stringent, but if he actually did run afoul of the prohibition, is he considered to have committed a transgression?

Arukh HaShulchan writes that one may not be stringent if all the halakhic decisors ruled leniently, but why really shouldn’t a God-fearing person be stringent on himself?

Yoreh De’ah, Arukh HaShulchan 116:25:
A dangerous animal, although permitted through slaughter, nevertheless the scrupulous are stringent with themselves not to eat it, and this has already been explained in section 17, see there. And an animal about which a sage ruled based only on his reasoning, and the law was not explicitly found that it is permitted, a conscientious person should not eat from it. But if the sage has a received tradition that it is permitted, he may eat it; all the more so if the permission is found in a book. And one who does not eat it is worthy of rebuke, unless there is a dispute among the halakhic decisors—in that case each person may be stringent on himself. But in a matter where all the halakhic decisors permit it, heaven forbid to be stringent on oneself, and this borders on heresy, as Pithei Teshuvah, subparagraph 10, wrote in the name of Shevut Yaakov, see there.

Elad,
I assume he would not be considered intentional, but rather inadvertent or coerced. This can be analyzed in light of the Rashba in Shevuot 18 regarding one who has relations with his wife close to her expected period and she then saw blood, who is not considered coerced—although one could distinguish between the cases.

Eitan,
He is not speaking about a doubt or a dispute, but about something agreed upon. In something agreed upon, there is no reason to be stringent.