Sadhana siddhas cannot fall anymore than any other kind of siddha (krpa-sidha, nitya-sidha). No one falls from Vaikuntha other than examples like Jaya Vijaya who come to this world for the purpose of facilitating rasa (in their case vira rasa). The baddha jiva on the other hand is “fallen,” and this materially conditioned soul can become a devotee and cause Bhagavan to enter his or her heart as a sweeper. He is so kind.
Regarding the difference between nitya-siddhas and sadhana-siddhas, there is no difference in that they are both siddha. According to Srila Bhaktivinoda Thakur (BVT), some souls emenating from Maha Visnu go “up” and become nitya siddhas, whereas others go “down” and become nitya-baddhas. These two would be constitutionally the same, either as sadhana or nitya-siddhas. However, there are different kinds of nitya-siddhas, and those constituted of svarupa-sakti like Nanda Maharaja, Subala, Lalita, etc., are constitutionally different from those constituted of tatastha-sakti that “originate” in Maha Visnu. Nonetheless, all siddhas are for all intents and purposes equal becasue they function equally, being motivated by pure love under the influece of Bhagavan’s svarupa-sakti.
Q. What determines which souls are going “up” and which are going “down”?
This is a good question. BVT answers in two ways: the jiva‘s free will and the will of Bhagavan. However, under scrutiny the first answer seems less satisfactory. For example, such a jiva‘s choice would be based on incomplete information (a partial view of either direction). With only partial information as to the nature of either direction, one’s exercise of free will is one that is not well informed. Thus how much can it really be an exercise of free will?
The second answer is more satisfying, although only with regard to those who go “down.” They facilitate Maha Visnu’s sristi-lila. Afer all, nitya-siddha jivas are also manifest from Sesa in Vaikunthna and Balarama in Vrakaloka for the sake of these lilas. So there is no need for them to come from Mahavisnu.
Incidentally, I have not seen this explanation of nitya-siddha jivas coming from Maha Visnu anywhere other than in the writing of BVT, who was perhaps the first to try to explain the position of the nitya-baddha jivas to the modern world and in doing so shift any blame for the suffering of the jivas away from Bhagavan.
The difference between jiva-shakti emanating from Sesa, Balarama and Mahavisnu is that those emanating from Balarama are suited for lila in Vrajaloka, whereas those emanating from Sesa are suited for lila in Vaikuntha. As for those emanating from Mahavisnu, they are suited for the srsti lila, a large part of which includes the opportunity for Bhagavan to offer salvation (Vaikuntha), and in the case of Mahaprabhu, the highest prema.