Does THC (Tetrahydrocannabinol) have anything to do with the level of DHT and hairloss?

You need to see a doctor for your scalp pain and redness. It is a sign of inflammation. You can guess on the Internet but it will not be your diagnosis.

Finasteride does not stop hair loss. It is a drug that is used for treating androgenic alopecia in men. There is no cure for this.  Finasteride can sometimes stop hair loss but it is not a permanent solution.  You need to use the medication for at least 6 months to see some benefit.

Laying upside down will not help with hair growth. Reading about these things on the Internet does not mean it is true.

Stop bleaching your hair if your are seeing hair falling out in clumps. The harsh chemical is likely causing the hair loss.

If your doctor recommends a scalp biopsy then you may need a scalp biopsy. Scalp pain is not normal.  A biopsy may not be the ONLY test you need but is is one of the many test you may need to determine why you are having your issues.

You need to find out why you are losing eyebrows before thinking about what medications to take. See a doctor first.