What is the ruling on a husband-to-be sitting alone with his contracted wife, before the consummation of the marriage, in a place where they are in khulwa shari'iyya (impermissible seclusion)? And does the marriage contract become void if the husband apostatizes after this seclusion?

A valid seclusion (khalwa sahihah) is one where sexual intercourse can ordinarily take place, meaning the spouses are secure from the sight of others, such as their seclusion in a house or a room, even in a tent in a desert, or a bridal canopy. If this seclusion is free from the sight of others, it is valid, and the rulings of consummation (dukhul) are established by it. It appears that the scenarios mentioned in the question fall under the category of valid seclusion, which establishes the dowry (mahr) according to the majority of scholars, and the right of return (raj'ah) according to the Hanbalis. The seclusion that is considered equivalent to consummation has the rulings of consummation applied to it in all matters, except for returning to a husband who divorced her three times, and in cases of fornication.