Can a person who declares their conversion to Islam remain a disbeliever if they have objections and doubts about certain matters such as what the right hand possesses (slavery), magic, jinn, and stoning the apostate?

Religion is based on absolute submission to the commands of Allah and His Messenger, even if the wisdom behind them is not fully understood, though most of the issues raised in Islam have known wisdom and clear proofs.

Regarding the issue of "slave ownership" (milk al-yamin (definition)), Islam came to reduce slavery by encouraging manumission and making it an expiation for several sins, and by elevating the status of slaves and prohibiting their degradation.

As for the "apostasy penalty" (hadd al-ridda), it is for the preservation of religion and society, to prevent doubts and discord that may arise from apostasy, and to protect people from being swayed by Satan.

Concerning "magic, jinn, and possession," these are real and known phenomena in all nations, and the sacred texts affirm their existence, and reason does not preclude the existence of unseen creatures.

Finally, the role of the intellect is to believe that Islam is true, and thereafter, it must submit to revelation. It has no right to object to Sharia (definition) rulings once it has acknowledged that the Sharia is true. It is incumbent upon a Muslim to seek the wisdom behind rulings after their legal establishment, and not to claim to be more knowledgeable or wiser than Allah.