Given that the standard is “do your best”, there’s a lot of flexibility to how to meet the requirements, given the complex circumstances. I would try to aim for something like the following, in your situation.
Since the pack isn’t meeting, organize a patrol of siblings or better still work with the other den leader to organize a joint “den” meeting, with the AoL-level Webelos serving as your den “patrol leader”. Conduct the various parts of the Scouting Adventure. Come up with a patrol name. Design and prepare a patrol flag. Create and practice a patrol yell. See if one of the older scout siblings could act in some ways as a “den chief” to help teach some of the symbolism of the First Class badge, how advancement in a troop works and differs from advancement in the pack, what scouts’ roles are in a troop and how those differ from leadership in the pack. Arrange to “visit” your friend’s troop eMeeting to see how that unit is conducting business under these extenuating circumstances.
I would approach this (and present it to the scouts) as an opportunity to learn about adapting to difficult circumstances and application of the scout motto. Who knows, this might even motivate the other den leader to undertake meetings with his or her own den separately to continue their Adventures.