How to exit early from a TSL node?

Questions
2

The standard way to do this in TSL isn’t to “return early” like you describe, but rather to conditionally assign a local variable, and then return that variable. You can turn a node into an “assignable” local variable in the shader by calling node.toVar():

const colorNode = Fn(() => {
    const returnVal = vec3().toVar();
    If(active.not(), () => {
        returnVal.assign(texture(texture, uv()));
    }).Else(() => {
        // I'm assuming "finalColor" comes
        // from some other scope
        returnVal.assign(finalColor);
    });
    return returnVal;
});

You can see this pattern repeated all over in the three.js library.
For example, in ParallaxBarrierPassNode (check out the parallaxBarrier node):

class ParallaxBarrierPassNode extends StereoCompositePassNode {
    static get type() {
        return 'ParallaxBarrierPassNode';
    }

    constructor(scene, camera) {
        super(scene, camera);
        this.isParallaxBarrierPassNode = true;
    }

    setup(builder) {
        const uvNode = uv();
        const parallaxBarrier = Fn(() => {
            const color = vec4().toVar();
            If(mod(screenCoordinate.y, 2).greaterThan(1), () => {
                color.assign(this._mapLeft.uv(uvNode));
            }).Else(() => {
                color.assign(this._mapRight.uv(uvNode));
            });
            return color;
        });
        const material = this._material || (this._material = new NodeMaterial());
        material.fragmentNode = parallaxBarrier().context(builder.getSharedContext());
        material.needsUpdate = true;
        return super.setup(builder);
    }
}
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