Toads do not become lonely or depressed. The lethargic behavior suggests a health or husbandry issue- possibly the same issue that caused the other toads death.
I don’t understand what you mean when you say that the toad melted.
The lump could be a cyst, or something else. It sounds like a visit to a qualified herp vet is in order, along with a review of proper husbandry for this species.