In summary: your issues was two fold. The loss of the Gui caused by luci-app-ntpc, and the log error messages cause by the install of ntp-utils - 4.2.8p15-4. The error message below indicates that two ntp pid, or programs can not work together.
Once the offending packages were removed you are left with the default package ntpd - 4.2.8p15-4.
The rc.local file was then populated via LuCi or file system with the suggestion for using static ips to sync time over the wan interface upon boot of your system.
If you still have ntpdate installed, this is exactly what catches time correction first on my system; as depicted in the screenshot on the two thread you have viewed.
To be sure, do not confuse one with the other. Nothing is needed for ntpdate to function. Your modified rc.local is using ntpd
ntpd - 4.2.8p15-4
ntpdate - 4.2.8p15-4
If you still have ntpdate installed look at your system log and follow the time stamp changes to the first drastic switch. You'll see ntpdate did the adjustment first and several lines down. ntpd is called into play to do it again (request time and sync). That is two separate process that will not conflict with one another.
So, if the above is understood to be correct, you can proceed to reinstall the WireGuard interface and proceed with more confidence.
Lastly, if this is truly the resolution to both your threads, please consider marking the issues as solved found in this topic [Solved]. See How to mark a topic as [Solved] for a short how-to.
Thank you for the challenge, and don't forget to write. 