How can we reconcile the Shafi'i condition of puberty and sanity for the sunnah of sacrifice with their view that it is permissible for a guardian to sacrifice on behalf of his wards from his own money, so that the reward of the sacrifice accrues to them?

There is no contradiction between stipulating maturity and sanity for the sunnah (definition) of the udhiyah (definition) and its validity on behalf of the minor and the insane, if it is done from someone else's property and the reward is designated for them. The udhiyah is a confirmed sunnah for the free, mature, sane, and capable Muslim. The guardian's sacrifice from his own property on behalf of his wards (such as a minor or an insane person) is valid, and its reward accrues to them. However, he is not permitted to sacrifice from the ward's property, because the udhiyah is a donation, and the guardian is prohibited from donating from the ward's property.