Something here may help.
https://support.microsoft.com/en-us/topic/the-msconfig-startup-selection-reverts-from-normal-to-selective-startup-4b506ba7-961e-8d4f-4bf0-51a0a98aaf56
To the other issue, not a lot to go on but the simplest solution may be to stand up a new one for replacement.
I'd use dcdiag / repadmin tools to verify health correcting all errors found
before starting any
operations. Then stand up the new 2016, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to again verify health, when all is good you can decommission / demote old one.
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