You need to know if the Tea Tree oil is steam distilled, expeller pressed or solvent extraction sourced.
Never EVER use any essential oil undiluted on your skin. You will develop an allergy to it with enough exposure. The best oils to dilute it with are extra virgin olive oil, organic virgin coconut oil (which are both really nourishing for your skin) and organic grape seed (sticky), or other organic oils. Mineral oil is a petroleum product, and it seems you said you couldn't use petroleum products.
If you are allergic to weed or tree pollen it does not automatically mean you are allergic to the oils derived from the tree or herb. Oils will not affect you the same way, as there are no histamines in the oil, the pollen and mold is the irritant in seasonal allergies. As I said before you can develop a contact allergy to any concentrated oil, and rarely a person can be sensitive on their first exposure.
The itch from scabies is caused by the little blisters that form on the skin. The blisters are scabie poop, and your skin is reacting to it. Scabies generally gravitate to warm areas such as between fingers and toes.
Anything with MSM added will be carried through the skin, it's a transdermal carrier. If your lotion has MSM but also has urea's (bug poison to kill bacteria) or dimethicone (for 'slip' so lotion is silky) that will be pulled through your skin and into your system, so be very careful about what is in that lotion. In my business I made mine with no chemical preservatives or fillers for that reason. You have to be very careful about things like that.
Were you into poison oak/ivy? If so, colloidal silver will clear that up quickly. Also gerardia. It's an antibacterial, antiviral and antifungal.