My players are planning a heist. Two of the players each have a Necklace of Adaptation and do not need to breathe. They want to go inside a bag of holding (which is allowed from what I know), then give the bag to the Druid who wild shapes into a rat and chooses to "merge their equipment into their new form". Thus allowing the rat to carry the two PCs into the mansion easy-peasy. This is super creative, but would that be possible?
As long as the players can fit through the neck of the bag they can exist inside until you're nice enough to let them out, thanks to the necklaces of adaptation.
Even without them, it would still be possible but how long they can hold their breath would then become an issue.
Just don't die because no one else will know to let them out...
I think using Bags of Holding like this is always a great idea. My players have used a BOH to fly team members across chasms, hide them from magically tracking hunters, sneak extra people into a party, etc.
It can subvert a lot of what the DM thinks might be a challenge for the players and turn tricky situations into slight inconveniences so some DMs may place limits on what can be done.
Yes, this is possible. Characters may enter a bag of holding, they have solved the potential problem of running out of air, and there is nothing about a bag of holding that would prevent it from merging with the druid's wild shape form like any other magical item or piece of equipment.
I would carefully track the weight of the two characters going into the bag to make sure they do not exceed the limit, but apart from that limitation, the plan seems solid.
Perhaps they're rendering moot some hard work you did on the mansion exterior and defenses, but this is a fun and creative idea that shows your players are engaged and having fun. This should be encouraged.
As a side note: this answer is valid in every edition of D&D and in Pathfinder 1e and 2e, as well. There may be some limitations with the Bag of Holding's capacity from edition to edition--it is only 200 pounds in D&D 4e, though Handy Haversqack holds 1,000--but as long as the weight limit is not exceeded, this plan is feasible in every edition plus both Pathfinder editions.