Thats why Infra as Code is very very important.
Sure it can help, but it's just not a one fix solution people thing. If you want a good test of your IAC, just provisioning a brand new environment first time using only your iac.
Huh? This is a strange assumption to make. Is your premise that IAC can't ever be truly reproducible?
If you are modifying things manually then you're not doing IAC.
The second rule of IAC club is that you do not hand modify your infrastructure.