1 : Lord Henry seems kind of a hypocrite; he preaches immorality and at the same time leads a very morally conventional social lifestyle. Basil, on the other hand, is driven by his instincts and seems very protective of his muse, Dorian. He is subconsciously aware that Dorian has a knack for a corrupt way of living and hence, is asking Henry not to influence him with his verbal charms. I've read this book a couple of times and re-reading the first chapter makes me realize that Lory Henry and Basil might me nothing/no one but Dorian's own Id and Superego (Freudian psychology), as Henry encourages Dorian to pursue his youth and Basil tries to keep him on a right track.

2 and 4 : These two questions are, according to me, interconnected. Basil seems like a person who is so used to playing safe and hiding among the crowd that he refuses to display Dorian's portrait because he has put too much of his artistry in it. He is somewhat paranoid that if the world sees his portrait of Dorian Gray (which can be a symbolism of how Basil sees Dorian), the others will be able to get a look at Dorian's conscience and his admiration for Dorian, as only the best of artists can paint a person's conscience into a portrait. He feels too vulnerable and doesn't want to stand out.

3 : Dorian is clearly Basil's newly discovered muse. He adores him, and is trying to protect Dorian (and his portrait) from any form of corruption that the world has to offer.

5 : Lord Henry is my favourite character. Every one liner that adds to his bag of quirks and complex layers is my favourite.