The causes of Hole in the head or head and lateral line erosion (HLLE) are greatly debated. Some have identified fungal sources, some say its environmental. The most common reasons for it (so we think) are poor water quality (either dirty water or high chemical concentrations out of the tap), lack of a high quality diet, lack of essential phosphates in the water, and a plethora of other reasons; you can google HLLE and see what they say about it. This is not to say that any of these reasons are concrete, some fish are very prone to HLLE while others seem almost immune to it.
The shelf life of melafix should be printed on the bottle with an expiration date and lot number. All the bottles i have bought off the shelf have been well in range of the date though, normally about 2 years till expiration.
Keep in mind that melafix is not actually designed to treat Hole in head or HLLE as there is no surefire cure. in fact it is thought that there is no cure at all, just trial and error methods to reverse the progression of the disease. IMO melafix has helped the healing of the lesions caused by HLLE in the fish that i have treated. Aquarium salt can be added though it should be used in moderation, many recommend using 1/2-1/4 of the recommended dosage. Some people swear by epsom salt baths to reverse the effects of HLLE.
You're main concentration right now should be vitamins and/or an increasingly varied diet plus LOTS of water changes (about 1/4 or 1/3 everday, if possible). A higher temperature is good too, try 82-82 degrees f. Remove active carbon from quarantine tank and make sure there is good oxygenation and water movement.
Good luck and let us know how it works out!