Is the Maghrib prayer valid if the worshiper intended to shorten it by mistake and then completed it after being reminded, and is it obligatory to repeat it along with the 'Isha prayer that was combined with it?

Maghrib prayer is not shortened by scholarly consensus. A prayer does not become a single rak'ah except for Witr, which is the Witr of the day. If Maghrib were to be shortened, it would become one or two rak'ahs, which would be an injustice and invalidate most of it.

If someone shortens Maghrib prayer out of forgetfulness and is alerted, they must perform one rak'ah and prostrate for forgetfulness (sujud al-sahw); their prayer is valid. However, if someone performs the taslim with the Imam, knowing of the deficiency, it is not permissible for them, and their prayer is invalid. If someone performs the taslim out of ignorance of the deficiency or the ruling, they do not need to repeat the prayer. The prayer of someone who spoke to the Imam is valid if they were ignorant of the ruling, based on the hadith of Dhul-Yadayn.

The prayer is not invalidated by the intention to shorten what cannot be shortened out of ignorance. However, if one intends to shorten it while knowing the ruling, their prayer is not valid from the outset.