[pax]

делай так

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public GameObject respawnPrefab;
    public GameObject[] respawns;
    void Awake() {
        respawns = GameObject.FindGameObjectsWithTag("Respawn");
        foreach (GameObject respawn in respawns) {
            Instantiate(respawnPrefab, respawn.transform.position, respawn.transform.rotation);
        }
    }
}

по второму вопросу - используй MonoBehaviour.OnTriggerStay