If i were you, i'd try a couple of chargers, the charging port is exactly the same across all surface pro's. So i don't see why it matters if you use pro 3 to pro 7 chargers. Having said that i dont understand why the charging light interacts differently for your various trials. Maybe some chargers are OEM ? dunno .
But all is not lost, if cannot turn on at all, i would just rip open the device and pull out the SSD and put into external drive enclosure. Then access data using another comp. (see the link, the drive is the standard size and can be removed, its not soldered). I won't even bother to pay money to repair it , pro 4 is going long in the tech cycle liao.
https://www.ifixit.com/Guide/Microsoft+Surface+Pro+4+SSD+Drive+Replacement/60383