Is it permissible for a wife to masturbate her husband with her hand, and is it permissible for the husband to externally stimulate his wife's clitoris using a vibrator until she reaches orgasm, as foreplay for lawful intercourse involving penile penetration, in order to satisfy the wife's desire in the event that the husband suffers from premature ejaculation before her satisfaction?

Based on the permissibility, there is no difference between a man masturbating with his wife's hand and a woman masturbating with her husband's hand and being aroused by him in the manner mentioned in the question. This is due to the absolute nature of the exception in His Almighty's saying: "And those who guard their private parts Except from their wives or those their right hands possess, for indeed, they are not to be blamed. But whoever seeks beyond that – then those are the transgressors" (Al-Mu'minun: 5-7). The verse encompasses all forms of sexual enjoyment between spouses, except what is specifically forbidden by a particular دليل (evidence), such as intercourse during menstruation or in the anus. As for the Quranic evidence on the prohibition of masturbation, it includes in its prohibition all means of sexual arousal, and the reason mentioned for using those stimulants does not permit the use of those artificial means.