Is the good end of one of the people of the innovated groups proof of the truthfulness of this group, the soundness of their path, and the obligation to follow them?

The judgment on the methodology of groups is not based on the good ending of their adherents, but rather by presenting their methodology to the Qur'an and Sunnah. Allah may grant a servant a good ending due to their repentance, or good deeds, or their sincerity and eagerness for the truth. And one may be granted a good ending even if they were upon error or innovation, or even if they were upon sins and major transgressions, as in the Hadith: "Indeed, a man may do the deeds of the people of the Fire, until there is nothing between him and it except an arm's length, then the decree overtakes him, and he does the deeds of the people of Paradise, so he enters Paradise." Thus, a good ending does not necessarily indicate the soundness of what a person was upon.