[oldman]

First I'd consider repotting. You probably don't know how much of the "blue stuff" is in the soil. I'm assuming it's Miracle-Gro for acid loving plants or something similar. If it's got crystals of fertilizer on the surface you probably can't flush it out with lots of water. Leave the rootball intact, but remove the loose soil. Aphid and spider mites don't mean you have problems in the soil too. The fertilizer is probably too much for critters too, even if it hasn't killed your Muppets, it's probably stressing them. Disturbing the rootball will only stress them more.

The citrus mix is best. I wouldn't add the fertilizer unless there was something to indicate you need it. It sounds like too much fertilizer is part of the current problem. You want good drainage and you want the soil to be able to dry out some between waterings. Peat wouldn't be a useful addition. You might want to consider airlayering the grapefruit to make them fit better in your growing area. If you decide to do that I'd recommend just scraping out the top inch of soil in the pots and replacing it with new citrus mix. After you get root growth on the part you're going to cut off you can pot the clone and wait for new growth on top before you repot the parent plants with new soil. That spreads the stressor out so the plant doesn't get hit with all of them at once.

Your approach to aphids and spider mites is good. Soap and water and gentle scrubbing work better than chemicals. Just remember to give the plant a gentle rinse and don't use lots of soap. It's to get the critters off. They will mostly be on the undersides of leaves, but they can be on the stems and trunk of the plant too. Repeat the wash in about a week to get rid of eggs or hatchling you missed the first time.

Ideally you'd like a quarantine area for new plants, but we work with what we have. Since your space is limited pay close attention to your existing plants to make sure none of them get mites or aphids from the newbies.

And good luck. One bloom on a citrus tree will make your whole place smell amazing. Since they bloom in winter the make a nice houseplant if you can keep them happy. Leaf curl is stress, not necessarily drying out too much. The tend to drop leaves occasionally, but should grow more than they drop. Fruit takes a long time to mature and probably won't taste too goo without natural sunlight. I'd recommend just picking most of it off when it's small. Lei the plant put its energy into staying healthy.