How to cure this sick plant?

It seems to me, Lyn, that your Leucothoe (otherwise known as 'Doghobble') had become thoroughly potbound at some stage. I had a similar problem with mine although not so serious. I removed it from its pot, teased out the roots and repotted it. It's now in an 18in pot. It took a very long time to recover, but it flowered this year.
I'm a bit dubious about feeding it with the Miraclegro. In its sickly condition, your shrub will not be able to assimilate it. There should be enough nutrient in the new compost.
Be patient, Lyn. I'm sure it will recover,but it will take time. Don't let it get too much sun, and make sure it doesn't dry out.

I can tell you what I did years back for a sick plant which I was told to burn. I had some commercial mentha arvensis v. piperascens(4% volatile oil content with 85% menthol-superb aroma!) and it came from the supplier full of rust and looked ready for the bin. An old gardener now sadly passed away, told me to soak the whole plant especially the roots in any fungal compound for 24 hours, but mints roots are large. if your plant roots are delicate and small soaking those for too long would kill it. I wouldn't soak the whole plant more than 8 hours and repeat if necessary.We used Dithane.

Now the plant has been thriving ever since and even the orange rust from a neighbour's roses does not affect it. This procedure sort of gave the plant's genetic makeup some immunity because subsequent clonal meristems resist rust now and other fungal spores. However your plant looks like it has had a battle with ph or too much lime and potbound, or even over feeding as concentrated nutrients may scorch the plant, so my tip may not be relevant.

The only thing I'll add is, did you pull out the roots from the ball? If they're solidly pot shaped and haven't spread out, you should tease out the roots so some are sticking out wherever possible, so that they grow and spread in the compost/soil once it's planted. And that's true of all contained plants, whether they're new or not, when you plant. You often get that "everything coiled around itself in the shape of a pot" thing in new plants from the garden centre - these should be teased apart and loosened before planting.