The difference between an artificial raised bed and the forest is that the forest adds organic matter at a slow rate, to the surface, and it takes years and years for an inch of additional topsoil to form. During that time it's also naturally eroding downhill when it rains. In your case, you added a lot more organic matter at once, and dug or tilled and mixed things up. This has the effect of fluffing the soil with air. It will settle a bit over time. In addition, the compost you add will continue to decompose. Only the mineral portion of soil will be permanent. It's fairly common for this to happen with new raised beds, and the effect is worse the more organic matter you used. I know there's another thread on this just in the last week or two, I'll try to find it.

Do what you can do without rotting their trunks with soil piled against them or killing the trees by cutting off their atmospheric moisture by filling beneath them. This probably boils down to a bare minimum layer of sandy or otherwise extra porous soil that will give you enough rootless ground to get underplantings started, until the trees root up into. Be sure to water enough to keep the orginal soil beneath the fill moist.

Ive personally never done anything like that myself. Maybe look in to some type of simple french drain of sorts, im sure if you google it you'll get tons of ideas. I think the big box stores usually carry ready made french drains that you just dig a trench for, put in place and bury... I like your idea of diverting to a rain barrel, but it would have to sit downslope of the garden and you would have to pump the water back up when you want to use it. let us know what you come up with.

How long have they been planted there? If it's been a while, it might be time to renew the acidity of the soil. Our water is alkaline and it slowly changes the soil back to the basic side. You can add pure sulfur or a brand name soil acidifier and it'll take a few months, but the acidity will return. You may read about other ways to quickly get it acidic again but I wouldn't risk most of the methods. Some ironite or similar fertilizer might help with it too since the actual problem with higher pH is less iron or manganese (frequently confused with magnesium) uptake. The tips browning might be from salt burn. It's usually not very serious, but it does affect the look of the plant and it can get worse over time. You can wash out the soil by giving the plant a lot of water all at once and let it wash out the salt in the soil. (I save rainwater for this purpose)