ネストしたコルーチンを止めてみる。

NestCoroutine.cs

using System.Collections;
using UnityEngine;

public class NestCoroutine : MonoBehaviour
{
    private IEnumerator _coroutine;
    private int _count;
    private int Count
    {
        get
        {
            return _count;
        }
        set
        {
            _count = value;
            Debug.Log(_count);
        }
    }

    private void Start()
    {
        Count = 0;
        _coroutine = MainCoroutine();
        StartCoroutine(_coroutine);
    }

    [ContextMenu("StopCoroutine")]
    private void StopCoroutine()
    {
        Debug.LogWarning("Stop");
        StopCoroutine(_coroutine);
    }

    [ContextMenu("RestartCoroutine")]
    private void RestartCoroutine()
    {
        Debug.LogWarning("Restart");
        StartCoroutine(_coroutine);
    }

    private IEnumerator MainCoroutine()
    {
        for (int i = 1; i <= 5; i++)
        {
            yield return new WaitForSeconds(1);
            Count = i;
        }

        yield return StartCoroutine(SubCoroutine());
        yield return SubCoroutine();

        yield return new WaitForSeconds(1);
        Count = 99;
    }

    private IEnumerator SubCoroutine()
    {
        var last = Count + 5;
        for (int i = Count + 1; i <= last; i++)
        {
            yield return new WaitForSeconds(1);
            Count = i;
        }
    }
}

結果

yield return StartCoroutine(SubCoroutine()) だと SubCoroutine が終わるまで止まらない。
yield return SubCoroutine() だと SubCoroutine の途中で止まる。

入れ子のコルーチンを StartCoroutine で再開するのは無理そう。