A: Only if it came from TP-Link (via Wayback) or OpenWrt. Scan it with VirusTotal before flashing.
A: No. It is 2.4GHz only (802.11n). Max speed is 300Mbps theoretical, ~80Mbps real-world. Have you successfully revived a TL-WR850N v3? Let us know in the comments below. Or, tell us your horror story about bricking it with the wrong firmware. tp-link tl-wr850n v3 firmware download
However, these old workhorses don’t just die. Maybe you are setting it up as a repeater for a guest network, or you just want to squeeze one more year of life out of it. To do that, you need the latest firmware. A: Only if it came from TP-Link (via Wayback) or OpenWrt
A: No. You will brick it immediately.