Regarding your first question, a pure devotee is never under the modes of nature. In other words, no material laws apply on a devotee because he is fully under the direction of Krsna. So a devotee can continue his present occupation or he may change it if he likes. Krsna does not force a devotee because the devotee acts spontaneously according to the desire of the Lord; in this kind of loving service there is no question of force. Force is applied only when there is denial of Krsna. Just as the citizens of a state have freedom to follow the laws of the state, that is subordinate freedom, and the freedom of the living entities is also subordinate to Krsna. The clue is given there in the Bhagavad-gita that the living entities are fragmental parts and parcels of the Supreme Lord, this means that all the qualities of Krsna are therefore present in minute quantity in each of His parts and parcels. That is simultaneously one and different philosophy—the living entities thus have free will because they are parts of Krsna and Krsna has free will, but the free will of Krsna is Supreme while the freedom of the parts and parcels is minute. So if the living entity out of love subordinates his free will to Krsna that is his liberation. He is no longer forced to act helplessly, but he acts freely rendering loving devotional service to Krsna in every way. Then we have the practical conclusion from Krsna confirmed by Him in Bhagavad-gita (18:66), "Just surrender unto Me; and in return I shall protect you from all sinful reactions. Therefore you have nothing to fear." In this way astral influences are also material and therefore they do not affect the devotee who has taken protection of Krsna's internal energy, by surrendering his life and soul in the service of the Lord.
Your second question about spacemen from other planets; it is stated in the Vedic literature that there are many planets where the inhabitants are more advanced than the inhabitants of this earth planet. So it is not unlikely that such people may have developed space travelling methods. They occupy higher posts in the creation of the Lord and so they may be considered as demigods in the same way as the president is specially empowered by the nation, but this does not mean that such spacemen are necessarily carrying the messages of the Lord, just as the business of the state employee is not to act as the state representative but he acts in his particular job.