Hi, and welcome to the Highway!

Let me just dive in, but as you've written you may already have heard a lot of this already.

Leprosy represents death (because the skin appears to be rotting away), and death is the result of sin. Menstrual blood also represents sin, perhaps because of its association with death. But why was Christ allowed to touch lepers without having to undergo the ceremonial cleansing?

Because He cleansed the leper - making him no longer unclean. Only the sinless God-Man could touch the unclean without being defiled. Elisha could not touch Namaan to cure his leprosy, but sent him to wash (2nd Kings 5). I think this is a picture of the holiness and divinity of Jesus, that He would not be made unclean Himself by the presence of or even contact with sin. He lived among sinful people. He came to make them holy like Himself. He has made holiness "transferable" from Himself to others, just as uncleanness was transferable from lepers/ As sin and death is transferred to us from Adam, so life and righteousness is transmittable by Christ, the "second Adam" (1st Corinthians 15:21-22).

Hopefully this is at least a start towards answering your question...

-Robin