Hope your medical issues get resolved

Will not be refunded as a DONE4 price is irrespective of he number of segments (that's why they are good value).
Is it a DONE4 or another RTW ticket ? Without checking I thought NOU was not possible on xONEx
You may be able to get taxes refunded less a service fee.
You will need a re issue as you cannot skip segments.
Trying to skip the continental LIM-IPC-SYD (or LIM-IPC-SCL-SYD?) will be the problem. Ask your insurance to fly you back to Chile.
Check with who ever issued your ticket. Its them, not FT, who will decide