Re: liver flukes?

  The NOW brand one ounce of 100% neem oil I have says "DO NOT INGEST THIS PRODUCT. Keep away from sensitive areas. Keep out of reach of children".  It's for lotion or shampoo.

There have been cases of poisoning in children, and it causes a temporary infertility in adults. Neem oil is ingested by men in India for this exact purpose. 

The other option would be to use the leaves, but I don't know if this will work on liver flukes. The leaves are used to kill intestinal parasites.

Raw pumpkin seeds are great for intestinal worms and eggs. This would be really safe and would work by killing off any eggs so they cannot reproduce. The adults will eventually die off.

Then there is black walnut hull that contains a number of active compounds including juglone. Black walnut is said to kill some parasites and it inhibits cellular division making it useful for parasites and malignant tumors.

What about just putting it on skin? Doesn't 80% of what you put on skin get absorbed?

This depends a lot on molecule size. Not everything will penetrate the skin. In fact the mucous membranes are more absorbant than the skin and neem oil is used intervaginally as a spermicide. So it does not appear that the oil penetrates the mucous membranes, which means it should not penetrate the skin either.