Is supplicating for all Muslims to enter Paradise or Firdaus without reckoning unlawful, knowing that some Muslims will be tormented first, or that the ranks of believers in Paradise vary? Or is it permissible based on "None of you truly believes until he loves for his brother what he loves for himself," and Allah does what He wills?

There is no harm in supplicating for Muslims to enter Al-Firdaws, unless the intention is that none of them should enter Hellfire in the first place. If that is the intention, then it is an excess in supplication, as it contradicts definitive religious texts. Scholars have differed regarding supplicating for the forgiveness of all sins of Muslims: some have prohibited it, some have permitted it, and some have said that the intention is to alleviate the effects of sins or to show compassion, not the forgiveness that prevents punishment, because there is a consensus on the necessity of the threat of punishment being carried out against a group of disobedient people.