Try doing a factory default reset first, configure just basic Wifi, then immediately proceed to flashing the new firmware. Your router might be crashing because you have settings that try to access the broken hardware acceleration system. If it still fails, flash it while in Recovery Mode.
Renewal worked for me a few hours ago when I turned on my RT-AC5300 for the first time in 6+ months, however the router didn't use the renewed certificate. According to your log so did yours, since it mentions the next renewal date is now in two months. I haven't had time to check any further, but I expect the new certificate would get used after the next reboot.
Either paste it on the Certificates page, or just re-import the .ovpn file from NordVPN.
No, I haven't recompiled it.