Is it safe to use the dental wax on the inside if the last bracket at the back of my mouth keeps cutting my tongue?

Yes: Even if you swallow this, there is no harm.

Yes, but ..: 1) you should not see this irritation for more than a day or two, so i suggest seeing your orthodontist to potentially make an adjustment and 2) i'd use a small amount so you don't ingest a large piece by accident (more likely on the inside of your mouth) 3) you do need to get used to some elements of braces, so after a potential adjust, do not rely on wearing wax all of the time.

Safe, yes: But contact your Orthodontic Specialist or treating General Dentist and ask for an adjustment. You can also take 1 200mg Ibuprofen every 4 hours, rinse with 1tsp salt in 8oz warm water, and dab (use Q-tip) Orabase-B onto sore spot on tongue.