In answer to your several questions, first; what is meant by conditioned sould? Conditioned soul means one who has accepted something illusion as reality. Conditioned means that due to imperfect desires the spirit soul becomes dependent on material conditions for his satisfaction.
Your second question; how does he become conditioned when the jivatma is eternal full of bliss and knowledge? The answer is that it is not the soul itself that becomes mixed with matter but it is his consciousness that becomes absorbed in trying to enjoy the matter. Out of desire to lord it over, the jiva soul forgets that he is eternal, full of knowledge and full of bliss and identifies with the material energy. So it is the consciousness of the conditioned soul that is affected.
Your third question is: does the soul ever become weak or strong on account of its association and contact with material nature? It is not the spirit soul that becomes weak or strong but it is his determination to become Krsna Conscious that me become weak or strong. By the influence of the modes of passion and ignorance he becomes weak and by the influence of the mode of goodness he becomes strong. But by the association with the pure devotee he becomes strongest.
Your fourth question: "Whether or not it is possible for the soul to exist without having this external gross and subtle body covering it? So if the bird is existing within the cage and someone takes away the cage does that mean that the bird can no longer exist? No. This body is illusion. It is only because of our attachment for this body that we have to remain within it. But as soon as one transfers his attachment to Krsna then no more is there any need for this body.
Your fifth question is, "Bhagavad-gita says that the conditioned soul is subjected to four defects. Does that refer to the body or the soul?" The defects are there in the living being when he is acting on the material platform. As stated in Bhagavad-gita chapter 15 verse 17: "dvavimau purusau loke, ksaras caksara eva ca, ksarah sarvani bhutani, kutastho 'ksara ucyate:" "There are two classes of beings, the fallible and the infallible. In the material world every entity is fallible, and in the spiritual world every entity is called infallible." So the defects are there only on the material platform. One who is acting in Krsna Consciousness is beyond those defects.
Your nest question: "The Bhagavad-gita says that Lord Brahma is a conditioned soul. Then how was it possible for him to impart the Vedas to a non-conditioned soul—Narada Muni?" So, Lord Brahma is a conditioned soul means that due to his desire to become the creator of the material manifestation he had to take birth within the material world. But although he was conditioned by such desire, still he is a great devotee of Lord Krsna. He has written Brahma-samhita which is pure Vaisnava literature describing the Supreme Personality of godhead. Lord Brahma was the first created being within the universe and the first spiritual master, as he received knowledge from within his heart from Lord Krsna. Narada appeared as the son of Brahma and therefore he had to accept Lord Brahma as his spiritual master.
Your last question, "How is the sould degraded (by lust, anger and greed) when it is transcendental to the body? Doesn't that imply that the sould has changed?" No. The soul never changes, but due to his tinyness he may become forgetful of his relationship with Krsna. The spiritual quality of the spiritual spark has not changed but he has forgotten his spiritual nature, bewildered by the imperfect desire to become equal or greater than Krsna.
I hope you are able to understand all these points. As you know, I am translating so many books into English from the Sanskrit, therefore, I am very busy. Now, I have representatives, the GBC and the temple presidents who are qualifies to answer your questions. Of course if you ask me I am obligated to reply because I am your spiritual master. So please direct your question to my representatives.
I may be coming to London in June, and I will be pleased to see you at that time.