You don't need to dig your roses up. Just topdress with the compost. The drip irrigation runs just under the mulch, so again, no reason to dig up your roses and totally set them back. If you only want to use organic methods, then of course you should not use a chemical fertilizer and insecticide combo. It's not terribly effective anyway. Just handpick and do the best you can. YOu can't grow perfectly spotless bug bite free roses without chemicals, so the first step in any organic program is learning to accept less than perfection. Learn to tolerate some damage. It'll be worse in some seasons than others. Do what you can with the tools you're willing to use, and don't sweat the rest. Roses are tough, as you've discovered. ANd, if you lose one flush, there's always another behind that one to take it's place.

As far as fertilizing, find some friends with horses. Clean the stalls. Horse manure makes great fertilizer. Ask them where they buy their alfalfa. Buy some for your roses. Alfalfa has a growth hormone in it in addition to being lots of yummy organic material for the soil microbes to break down. Place the manure and the alfalfa just at the drip edge of your roses 3-4 times a year and make sure they're watered well. Enjoy! That's all there has to be to it in a rose friendly climate like CA.

One of the best things you can do for your roses is to get a soil test and find out what type of nutrition your soil already has and what it's pH is. This is way important information and adding stuff without knowing that information would be like trying to make a cake from a boxed mix and assuming that all you needed to add was water, when it was brand X that required oil and eggs as well. Or, as in the case of the cake (and the soil), the even worse scenario of assuming that you needed to add everything from baking powder through the oil and eggs and flavoring when all you needed to do was add water. Some soils are already almost toxic in some macro and micro nutrients, and it's sure wise to know what you're starting out with to know where you need to end up.

The "Bristly Rose Slug" is the larva of a sawfly and digging the bed and replacing the soil won't do much to keep them away. A soap and water spray, 1 percent soap or about 1 teaspoon of soap per quart of water (and it should be real soap not dish detergent since it is the fatty acids in that soap that do them in) works very well.
These guys seem to prefer my Hibiscus to the roses since I've not seen them on the roses since I've had the Hibiscus.

The advice above is all good.

The worms eating the leaves are the larvae. They come from eggs laid by winged adults on the leaves. The winged adults come from pupae that overwinter after the larval stage. At least one kind pupates in the soil under the plants where the nematodes might aid in control. Probably you would only install the nematodes once.

However, if you kill most of the larvae on the leaves, there will be few pupae in your soil. Next year, patrol the plants and either wipe them off or spray soap on the affected area (under the leaves) immediately when you see damage. You will have to check back in two days to see if the soap is in fact killing them. No need to spray the whole plant-- they don't move from where the eggs were laid until the area is defoliated.

If I can re-emphasize-- the problem isn't infested soil. Rose slugs are normal in almost every rose garden, and the adults will seek out a new bed to lay eggs on the leaves. What is abnormal is the heavy damage you suffered. Usually it is minor as wasps and other predators pick off the larvae. If there are a lot of slugs and not enough wasps, you need to play that predatory role yourself. The slugs are sedentary on the chewed foliage and easy to find. In early stages they are quite small, though.

Thanks for the support - I am happy to report that the insecticidal soap is working well. Even the hibiscus is flourishing. There has been no further chewing on the one rose bush that was getting attacked during this "flush". Lots of buds. I'll just keep the spray handy - works well on ants too...