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I would cut back any dead plant material. Inspect your plant for any possible infestation. Sometimes just an overdue pruning may rejuvenate your shrubs.
If you decide to remove and start over, make sure you remove all old roots and plants before replanting your area.

Here is a link to proper care.

https://www.gardeningknowhow.com/ornamental/shrubs/juniper/growing-creeping-junipers.htm

Yes, you can propagate Creeping Juniper from cuttings.
Here is a link that will help you.

https://content.ces.ncsu.edu/plant-propagation-by-stem-cuttings-instructions-for-the-home-gardener/
https://www.gardeningknowhow.com/ornamental/shrubs/juniper/growing-creeping-junipers.htm

The poor growth is likely due to too much water and not enough sunlight.

Here is a link to refresh you on the care requirements.

https://www.gardeningknowhow.com/ornamental/shrubs/juniper/growing-creeping-junipers.htm

It could be a nematode infestation or a nutrient deficiency in the soil, but, without seeing the shrubs and their placement in relationship to each other, it's hard to tell what the cause may be. If the shrubs are valuable to you, you may want to get a complete soil test done. You can contact your local cooperative extension for information on where you can get this done.

I have taken some weed killer that I have mixed up and poured it into a plastic painting bucket, then using an approximately 2 inch paint brush, I paint it on the weeds leaves. It is a painstaking venture to be sure, however I get the weed killer where I want it without taking out or causing harm to my other plantings if the weeds get ahead of me.

Evergreens like creeping junipers often don't need fertilizer, unless they appear to have a nutrient deficiency. Here is some information on fertilizing evergreens:
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/fertilizing-evergreens-conifers/

There are different formulations of Miracle Grow, so it's best to look at the fertilizer analysis numbers on the bag and match them to the advice in the above article. This article explains how to read the fertilizer anaysis numbers:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fertilizer-numbers-npk.htm