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This is a dangerous subject. Do not use that soil for gardening. There is no good way to tell whether they have eaten any meat, or anything exposed to bacteria. Their feces is likely contaminated and should be treated as such.

I would recommend container gardening, using potting soil. This will help you to avoid any complications.

I wouldn't attempt to use this one. You will have to sacrifice, either, the bed, or the hedgerow.

The other alternative is to find a new spot and make it into a bed.

French Drains are very useful for this application. This article will help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/yard-drainage-problems.htm

Slime mold thrives in damp areas. Increasing air flow in an area or allowing the mulch to dry out will help. It has nothing to do with type of mulch or mulch size.
This article has more information.

https://www.gardeningknowhow.com/ornamental/fungus-lichen/what-is-slime-mold-slime.htm

The feces of cats can contain a multitude of parasites including the one that causes toxoplasmosis, a disease that can be dangerous to pregnant women or those with a weak immune system.
The second concern is that cats like to use areas that smell like their waste, so it's important to remove all sources of that smell, otherwise they'll be back.
Here is a link that will help you.
https://www.gardeningknowhow.com/plant-problems/pests/animals/cat-repellent.htm

I would take it back to wherever you purchased it no matter how long ago.

If you had opened it earlier and partially used it and the critters entered that way, I would just dump it all out somewhere far away in the backyard.