Although QueueExplorer makes queues and messages look similar to folders and files, under the hood they are still first-in first-out queues. For some operations QueueExplorer will perform Receive and/or Send operations. Send operation will create new message and they'll be put at the end of the queue.
Azure Service Bus can activate throttling if there are too many operations going on. Unfortunately, one of effects of throttling is that QueueExplorer can get partial list of queues and topics, without any error messages. In this case, it's impossible to know whether these queues were deleted or just temporarily not listed because of throttling.
As a workaround since version 4.5.29, QueueExplorer will use cached queues/topics if they are missing within 30 seconds. It will also always keep currently displayed node (queue, topic, subscription, etc) even if it's no longer listed by Service Bus. These workarounds are applied only for automatic refresh. If refresh is manually invoked, or as a result of operation like delete/add queue, QueueExplorer will show whatever Service Bus returns.
Deadlettered messages in a subscription can't be requeued to that subscription alone. That's because Azure Service Bus has no way to send a message to single specific subscription - messages can only be sent to parent topic, and then distributed to all its subscriptions based on rules. Therefore, if you want to requeue a message from subscription's deadletter, it will be sent to other applicable subscriptions as well.