Inoculation is done by injecting a pesticide into the soil at the base of a tree so it gets taken up by the roots and into the sap. When the adelgids drink the sap, they get a dose of pesticide and die. It's useful for individual trees, not so much for a forest, thus the efforts to find a predatory beetle to control the adelgids.

At Clingman's Dome, you've mainly got spruces and firs, which are being impacted by acid rain and another pest insect. The Park continues to treat those trees as well, though the firs have been pretty heavily impacted. I believe the main hope for the firs is in saplings and the seed bank.

There are a couple hemlocks in the oak forest near my house, and they've been adelgid free, but I visited one last week and it's infested.

It takes the adelgids several years to kill a tree, so it's not an especially urgent problem. Soap sprays suffocate the insects that get sprayed, so they are only good for immediate, one-time treatment. The insecticide persists in the sap for a while (Factchecker's two years sounds correct), so it has some preventive effect. You can probably wait until you see adelgids on the tree before treating it, as long as you inspect the tree regularly.

If you don't see little white puffs on the undersides of the needles right now, I bet you'll see them next winter. A young tree might be more vulnerable, so you might want to talk to a pro, but basically you can wait until your tree is infected.