If a person is going to do this method - I would suggest doing is a little differently. Have a tank set up - and feed it well in the separate tank until you're sure that you're not going to spread something to the rest of your fish. Once it's healthy - and doing well - you also have the choice to treat with hyposalinity, for example - and or prazipro - which would mitigate a lot of the quarantine risk you're mentioning. If one can not quarantine a fish, its probably best not to buy that type of fish - since depending on the source, you can risk the rest of your tank.It appears it was half gone when it was delivered. I dont think you could have done anything to help it. If your dead set on a leopard add a ton of pods to your main display before adding the fish. Then I'd skip quarantine and put it directly into the main display. Its not the most popular opinion to not quarantine first but the less stress on these fish the better. Its worked well for the ones in my tank.Does your tank have plenty of sand across the whole floor? A pile here and there probably isn't enough.
A large part of quarantine risk/benefit also depends on the source of the fish. If you have an LFS nearby where the fish can be monitored (which is not uncommon as a practice) - and be ensured that its eating - and its not going to be traveling across country, there may be less of a risk. Additionally, purchasing already quarantined fish is also a better option (IMHO) than just putting them in your display.
