I got the wrong impression of this thread reading the "tampering" in the title.... I'm relieved to find you're not messing with your prednisone dose as I first thought.
Most of the side effects will ease off as you taper, some may remain for a while once you're off, but it's highly unlikely any will be permanent. Cushing's, diabetes and osteoporosis you don't need to worry about unless you're on prednisone long-term or you already have other risk factors for these conditions, though some of the symptoms of Cushing syndrome can appear in a shorter time on prednisone, once you're tapering and down to a low dose, it's unlikely you'll get any new symptoms at this stage. It may be worth bearing in mind that the side effects you do have - skin, hair and mood changes - are things most teenagers acquire without the help of any medication, so if these things don't return to how they were for you before you started prednisone, they may just be part of adolescence. But whatever the cause, they can often be treated quite effectively, so it doesn't mean they have to stay that way. 5lbs weight gain is very little - prednisone can make you retain water, so it's possible that the weight is not fat at all, just the weight of extra water in your body that will go once you're off the medication. If it isn't, again if you're a teenager fast weight gain is entirely normal, necessary even, unless you're very overweight already. Even if you were already overweight, and the 5lbs is not water, prednisone increases appetite, but this also will normalise soon after stopping the prednisone and your weight gain should normalise too.
Have your tried any skin or hair treatments since you started having these problems? What is your relationship with your doctor like? The doctor who prescribed prednisone for you should have made clear you're not at risk of diabetes, osteoporosis, etc. from a short course - it was remiss of the doctor to cause you unnecessary worry. Never suffer side effects in silence. Some are inevitable, but many can be helped with treatment: e.g. I had terrible insomnia on prednisone but my doctor managed to find a sleeping med for me which helped so much. If any side effects trouble you, ask your doctor and post on this forum. There isn't always an answer but very often there is something that can help.