Spawning issues

I have a project that is essentially just an endless runner. I just recently have coded in the part that makes it so I cannot have two blocks spawn adjacent to each other that will block the path to the player.

I do so by creating prefabs for all 19 possible platforms: (0 denotes clear, x denotes pitfall and b denotes block)

 platform1: 000
 platform 2: b00
 platform 3: bx0
 platform 4: bb0
 platform 5: b0x
 platform 6: b0b
 platform 7: x00
 platform 8: xb0
 platform 9: xx0
 platform 10: x0b
 platform 11:x0x
 platform 12: 0b0
 platform 13: 0bb
 platform 14: 0bx
 platform 15: 0x0
 platform 16: 0xb
 platfrom 17: 0xx
 platform 18: 00b
 platform 19: 00x

Using this key I created 7 different "classes" that blocks fall into and which blocks cannot be adjacent to them.

 all clear: 
     contains: 1
     cannot adjacent: none
 left covered:
     contains: 2, 7
     cannot adjacent: 13, 14, 16, 17
 left & mid covered:
     contains: 3, 4, 8, 9
     cannot adjacent: 5, 6, 10, 11, 13, 14, 16, 17, 18, 19
 left & right covered:
     contains: 5, 6, 10, 11
     cannot adjacent: 3, 4, 8, 9, 12, 13, 14, 15, 16, 17
 middle covered:
     contains: 12, 15
     cannot adjacent: 5, 6, 10, 11
 middle & right covered: 
     contains: 13, 14, 16, 17
     cannot adjacent: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
 right covered:
     contains: 18, 19
     cannot adjacent: 3, 4, 8, 9

Using this information, I created 7 different GameObject[]'s that contain only blocks that are safe to the 7 areas. (i.e. in GameObject[] left_safe_walkways I have all platforms except for 13, 14, 16, and 17).

With all of this in mind, I created an IENumerator that currently spawns up to 100 platforms using a for loop. I have, since using code to limit the block spawns, have still been seeing impossible parts in the game. Is there anything I can do to fix it? Here's the code I am using currently: using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class spawnwalkway : MonoBehaviour {
     public GameObject[] all_walkways;
     public GameObject[] left_safe_walkways;
     public GameObject[] left_mid_safe_walkways;
     public GameObject[] left_right_safe_walkways;
     public GameObject[] mid_safe_walkways;
     public GameObject[] mid_right_safe_walkways;
     public GameObject[] right_safe_walkways;
     public int walkway_number;
     public int next_walkway_number;
     int plat_num_random;
     // Use this for initialization
     void Start () {
 
         StartCoroutine(spawnBlock());
     }
     
     // Update is called once per frame
     void Update () {
         
     }
 
     IEnumerator spawnBlock()
     {
         int walkway_number = Random.Range (0, 18);
         for (int z = 1; z < 100; z++)
         {
             if (walkway_number == 0) {
                 next_walkway_number = Random.Range (0, 18);
                 Debug.Log (all_walkways [next_walkway_number] + "block "+ z);
                 Instantiate (all_walkways [next_walkway_number], new Vector3 (-1.42637f, -0.4771186f, 2 * z + 8.574288f), Quaternion.identity);
             } else
             if (walkway_number == 1 || walkway_number == 6) {
                 next_walkway_number = Random.Range (0, 14);
                 Debug.Log (left_safe_walkways [next_walkway_number] + "block "+ z);
                 Instantiate (left_safe_walkways [next_walkway_number], new Vector3 (-1.42637f, -0.4771186f, 2 * z + 8.574288f), Quaternion.identity);
             } else
             if (walkway_number == 2 || walkway_number == 3 || walkway_number == 7 || walkway_number == 8) {
                 next_walkway_number = Random.Range (0, 8);
                 Debug.Log(left_mid_safe_walkways[next_walkway_number] + "block "+ z);
                 Instantiate (left_mid_safe_walkways [next_walkway_number], new Vector3 (-1.42637f, -0.4771186f, 2 * z + 8.574288f), Quaternion.identity);
             } else
             if (walkway_number == 4 || walkway_number == 5 || walkway_number == 9 || walkway_number == 10) {
                 next_walkway_number = Random.Range (0, 8);
                 Debug.Log (left_right_safe_walkways [next_walkway_number] + "block "+ z);
                 Instantiate (left_right_safe_walkways [next_walkway_number], new Vector3 (-1.42637f, -0.4771186f, 2 * z + 8.574288f), Quaternion.identity);
             } else
             if (walkway_number == 11 || walkway_number == 14) {
                 next_walkway_number = Random.Range (0, 14);
                 Debug.Log(mid_safe_walkways[next_walkway_number] + "block "+ z);
                 Instantiate (mid_safe_walkways [next_walkway_number], new Vector3 (-1.42637f, -0.4771186f, 2 * z + 8.574288f), Quaternion.identity);
             } else
             if (walkway_number == 12 || walkway_number == 13 || walkway_number == 15 || walkway_number == 16) {
                 next_walkway_number = Random.Range (0, 8);
                 Debug.Log(mid_right_safe_walkways[next_walkway_number] + "block "+ z);
                 Instantiate (mid_right_safe_walkways [next_walkway_number], new Vector3 (-1.42637f, -0.4771186f, 2 * z + 8.574288f), Quaternion.identity);
             } else
             if (walkway_number == 17 || walkway_number == 18) {
                 next_walkway_number = Random.Range (0, 14);
                 Debug.Log(right_safe_walkways[next_walkway_number] + "block "+ z);
                 Instantiate (right_safe_walkways [next_walkway_number], new Vector3 (-1.42637f, -0.4771186f, 2 * z + 8.574288f), Quaternion.identity);
             } 
 
             walkway_number = next_walkway_number;
             yield return new WaitForSeconds(0.5f);
         }
     }
 
 }