This may not be relevant, since the plants are already in the ground, but whenever i hear 'drainage issue", I think "raised beds".
If you could go back in time and make the bed deeper - perhaps even as deep as the surrounding stones allow - and mix the manure compost with faster-draining, coarse material, the drainage would be improved.
I've often wished for a gardening time machine after learning something! Good luck with these plants coming back. I have a few hostas in a patch of grass, left there by a previous owner. They come back even if I step on them or nick them with the mower.
(My favorite soil-lightening material is bark. I start with pine bark nuggets or cheap confer mulch and screen it to get gritty and chunky stuff for more air and faster drainage. I use fine and fibrous bark to hold more water, or to mix with heavy clay soil and give it a little structure for a few years.
Of course, grit, crushed stone and very coarse sand or Perlite will help drainage. And inorganic things won't decompose and subside next year, or require nitrogen as they decompose.
But I agree with the person who didn't want to dig down into soil that has tree roots in it! Better to build "up" if you can, and have a raised bed. Just don't let the soil bury the bark - it won't like that. You might put a small "inner wall" around the tree so its bark keeps the existing depth.
