Purple spot is somewhat misleading as the name almost suggests a disease, whereas the cause is quite different. The discolouration arises from the consequence of poor soil conditions and is accentuated by a marked deficiency of one of the more important of the trace elements: magnesium.
Plants should not go short of food, but it does not follow that you can grow better plants by forcing them to accept a super-abundance of food down their throats, and you are grossly overdoing it with this rate of fertilizer and foliage feeding.
I would cut out all feeding for at least an entire season and give the surplus a chance to weather and disperse. You will have to get the acidity reversed, but you should not try to do this quickly because a heavy application of lime, for example, will cause an even heavier short-term release of plant nutrients and lush growth that will be prone to attack by diseases.
Assuming that you will be planting at the optimum time, November, mix into this compost mix a level teaspoonful of Humber manure, or the organic nutrient feed that you intend to use. This quantity is very little as it is not to provide a nutrient residue at this stage - the John Innes content will do that - but to encourage a buildup in the bacteria strains that will later be involved in the decomposition of the organic manure you are going to use. Frequent watering to avoid drying out must also be expected to wash out soluble nutrient so much faster, hence the wisdom of using slow-release, long- lasting organic manures rather than chemical fertilizers.
The magnesium deficiency can be corrected quite quickly by making up a solution of Epsom Salts and mist spraying the foliage every other day, just enough to wet the leaf surface. I have found that a level dessert spoonful in a gallon of water is quite strong enough; the important point is the frequency. A drip or two of washing up detergent in the spray ensures a good wetting, and if it drips off into the soil it will do no harm.
The management of these tubs will be difficult enough with just a rose in each, and will be exacerbated by adding anything like other dwarf plants around the rim. Apart from the virtual impossibility of finding any plant that will enhance and blend with a rose, the extra moisture and nutrient absorption is just not worth it. Finally, bear in mind that tub-grown roses are subject to the same pests and diseases as those grown in the open ground, more especially if you let them suffer wilting and drying. Because of the relative size, a greenfly attack on a miniature could be devastating, so guard against this by starting a routine of precautionary spraying with a systemic insecticide every two or three weeks as soon as growth commences in spring.
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