"UMA Graphics" means the Intel HD Graphics 4000 in my case
Yes, that's what it means. UMA isn't a very common term. In this context it stands for Unified Memory Architecture, referring to how HD Graphics uses system memory instead of dedicated graphics memory. Here is Lenovo Support Specialist specifically confirming what this is.
The NVIDIA GPU will not actually turn on and generate heat/consume energy, just not be used
Yes, that's the purpose of switchable graphics and using HD Graphics. Also confirmed in the previous link.
The process is reversible/safe
Yes, it's reversible by reverting the same settings. I'm not sure what you mean by "safe". If you have an application that requires graphic capabilities beyond HD Graphics 4000, that application will run poorly or not at all.
I would recommend disabling it in the UEFI rather than the Device Manager. This should prevent a utility from re-enabling it. It would also disable it when you are outside of Windows.