Is the fiancée sinning if she accepts to marry a man whose religiosity is lesser, but towards whom she feels affection and compatibility of temperaments? And is she sinning for her aversion to a long beard, despite her acknowledgment that it is a religious symbol?

The Sharia encourages marriage to a person of good religious character and morals, and there is no harm in considering other aspects. You are not obligated to accept the first man who proposes, as the command in the Hadith is not for obligation. Be sure to perform Istikhara. Disliking a long beard merely due to natural inclination is not blameworthy. A suitor is a non-mahram to the fiancée, so leniency in interaction with him is not permissible. The best way to ascertain the suitor's character is to ask trustworthy individuals and those who know him closely.