Loadlevel.async stops at ~90%?

AsyncOperation.isDone will never be true if AsyncOperation.allowSceneActivation is false. Progress will stop at around 0.9f. You can work around this to do whatever needs to be done before the final load takes place by waiting until progress has reached 0.9f, and end with allowing the scene activation again after you've done what you need to prior to that. Example:

 using UnityEngine;
 using System.Collections;
 
 public class LoadLevel : MonoBehaviour {
     public string level;
     private string progress = "";    // Name of scene to load.
     private bool isLoading = false;
     private bool doneLoading = false;
     private bool allowLoading = false;
     private void OnGUI() {
         GUILayout.BeginVertical("box");
         if (!isLoading) {
             if (GUILayout.Button ("Begin Load")) {
                 isLoading = true;
                 StartCoroutine(LoadRoutine());
             }
         } else {
             if (doneLoading) {
                 if (GUILayout.Button ("Actually Load")) {
                     allowLoading = true;
                     StartCoroutine(LoadRoutine());
                 }
             }
             GUILayout.Label(progress);
         }
 
         GUILayout.EndVertical();
     }
     private IEnumerator LoadRoutine() {
         AsyncOperation op = Application.LoadLevelAsync(level);
         op.allowSceneActivation = false;
         while (op.progress < 0.9f) {
             // Report progress etc.
             progress = "Progress: " + op.progress.ToString();
             yield return null;
         }
         // Show the UI button to actually start loaded level
         doneLoading = true;
         while(!allowLoading) {
             // Wait for allow button to be pushed.
             progress = "Progress: " + op.progress.ToString();
             yield return null;
         }
         // Allow the activation of the scene again.
         op.allowSceneActivation = true;
     }
 }

One important thing to note is that anything that happens in Start or Awake on any of the MonoBehaviours in the new scene will not be performed asynchronously. It will only load the things that are in the scene before any code in it is actually run. Another example of an Awake/Start script in the loaded level that takes a long time to process shows that this is the case:

 using UnityEngine;
 using System.Collections;
 
 public class WorkAtStart : MonoBehaviour {
     public int calculations = 10000;
     public int iterations = 10000;
     public bool inAwake = true;
     public bool inStart = true;
     void Start () {
         if (inStart == true) {
             Calculate("Start");
         }
     }
     void Awake() {
         if (inAwake == true) {
             Calculate("Awake");
         }
     }
 
     string debugText = "Calculating...";
     private void Calculate(string funcName) {
         System.Diagnostics.Stopwatch stopwatch = new System.Diagnostics.Stopwatch();
         stopwatch.Start();
         Vector3 calc = Vector3.zero;
         // Do some random calculations:
         for (int i = 0; i < iterations; i++) {
             calc = Vector3.zero;
             for (int c = 0; c < calculations; c++) {
                 calc = calc + Random.onUnitSphere;
             }
         }
         stopwatch.Stop();
         Debug.Log (stopwatch.Elapsed);
         debugText += "\n" + funcName + ": " + stopwatch.Elapsed.ToString();
     }
 
     private void OnGUI() {
         GUILayout.BeginVertical("box");
         GUILayout.Label(debugText);
         GUILayout.EndVertical();
     }
 }
 

So if you need to do asynchronous things in Start/Awake you will need to fire up some Coroutines or do some threading inside the new scene.