Reinsert the long quill stem bolt and thread into the wedge, but leave it loose. Tap the top gently with a rubber hammer, then less-gently, with increasing levels of force till it moves. If no luck then use a steel hammer for the shock/impulse.
You can also remove the front wheel and flip the bike over, then add penetrating oils under the wedge. IE, from "below" and let them soak upward. Heat might help, but getting heat on the steerer tube and not on the wedge might be a challenge. It might be possible to get the quill bolt or some matching threaded-rod into the wedge from below, and then into a slide hammer
Your other option is to not remove the stem and ride it at the current height, which makes bearing replacement extra hard.