http://codereview.stackexchange.com/questions/128386/obstacle-spawning-and-movement-scripts/128403?noredirect=1#comment240340_128403

code fix.cs

#region fix #1

    // ObstacleSpawnerBase: change IEnumerable<WaitForSeconds> to IEnumerable
    protected abstract IEnumerable SpawnObstaclesImpl();

#endregion

#region fix #2

    // ObstacleSpawnerBase: change IEnumerable<WaitForSeconds> to IEnumerator
    protected abstract IEnumerator SpawnObstaclesImpl();
    
    // ObstacleSpawnerBase.SpawnObstacles: 
    IEnumerator SpawnObstacles()
    {
        yield return new WaitForSeconds(obstacle.startGameWait);

        while (true)
        {
            var etor = SpawnObstaclesImpl();
            while (etor.MoveNext())
                yield return new WaitForSeconds(obstacle.spawnWait);

            yield return new WaitForSeconds(obstacle.waveSpawnWait);
        }
    }
    
    // in the "core implementation": change IEnumerable to IEnumerator
    
#endregion

Sorry I was out of town today and didn't have my computer on me. So for clarification, the classes in the ObstacleSpawnerBase script do NOT have to be abstract? And the image under your sentence "So either group them under a single file:", the C# script name would be entitled "ObstacleSpawners"? And what do the "{ /.../ }" mean? Sorry I keep dragging this on I just want to learn as much as possible :) @Xiaoy312