Regarding the first question in your letter about how do we know of the spiritual abodes since once going there no one returns, you should know that the great liberated souls and incarnations who appear from time to time in this material world are not actually coming back, because they are never subject to material contamination or the laws of material nature. For the purpose of delivering the fallen living entities they come here temporarily and then go back when their business is finished, and this is all under the direct order of the Lord. So the appearance of the Lord or the great liberated souls in the material is different from the appearance of the contaminated living entity who is forced to take birth in the material world due to his desire to lord it over the world.
Letter to Vrndavanesvari, 6 June, 1969