What is the ruling on the act of rubbing the penis through trousers with the wife one is contracted to before consummation? And does it necessitate ghusl or wudu if it results in secretions? And is kaffarah obligatory, and must past prayers be re-performed if ghusl was obligatory and not performed?

There is no objection to the husband enjoying his wife immediately after the marriage contract. Regarding the discharges from the woman: if it is maniyy (seminal fluid), ghusl (ritual bath) is obligatory. If it is madhiy (prostatic/pre-seminal fluid), ghusl is not obligatory, but purification of whatever it has touched on the body and clothes is required, along with ablution (wudu'). If maniyy was discharged and she did not perform ghusl, it is more cautious to make up for the missed prayers.