I have a weeping Santa Rosa plum on Myro rootstock that seems to be doing fine. This is the second year in the ground and it has started to blossom. I don't now anything about Bud 9, but I use M111 Rootstock for apples, which seems to have some tolerance to wet soils. I would recommend to mulch as much as you can to help alleviate some of the drainage issues. Mulching every year will help break up the compacted soil allowing for better drainage.

I know that both Kalmia polifolia (swamp laurel) and Kalmia angustifolia (sheep laurel AKA lambkill) will grow in wet soil or even part-time very shallow water. IME both are smaller and less ornamental than Kalmia latifolia, mountain laurel, but mountain laurel likes moist well-drained soil. I think if your site is too wet for rhododendrons, it will also be too wet for mountain laurel.

If it's shaded by a maple, I am surprise that it's moist. Perhaps because it's raised. How dim is the shade? How long did you give for the hosta to be established. Small hostas are very slow growing and may take a while. You mentioned slugs, what's causing the current plant to fail? Paul

There are very few plant on earth that do not have symbiotic relationship one or more mycorrhiza. With synthetic fertilizers, most plant will do fine without any. Most plants, we refer to as annuals are grown in nothing more than sterile peat and perlite. It really comes down to how you want to grow your plants. Often plants, such as many succulent, that live in areas with very little nutrients in the soil depend heavily a symbiotic relationship mycorrhiza. The mycorrhizae gets what it needs, such as moisture directly from the plant. A plant that stores water is great place to obtain water in a barren region or a region with a wet and a dry season. Mycorrhizea might be the only thing able to make those nutrients accessible to the plant. Some fungi are strong enough to burrow right through hard rock. It's believed that, when the earth was nothing more than rock, fungi were instrumental in breaking that rock down to usable soil. Shortly to follow was blue green algae. A bacteria that obtains energy through photosynthesis. That lead to an atmosphere able to support early plant life. But, none of this was possible with out fungi unlocking nutrients found in rock. Here are a few pictures, I took this morning. I am pretty sure it is just mold growing on the surface that stays dormant, until I water the soil. I can also see something green growing on some of the pumice. Most be algae. Pumice does hold water. I ask the identification, only because I saw it growing not only on top of the soil, but all the way through the soil of the one plant I took out to investigate. I will not be giving a full watering until the weekend to most of the plants.