As long as Python does the right thing with globs, there is really no room for marcel to get it wrong. Not sure what additional validation you are thinking of.
The File object encapsulates a path. If you use it, e.g. to read contents, and the file doesn't exist, then it will fail with an appropriate error message.
Files come and go. References to them go stale. Every user and tool deals with this. This isn't a "validation" issue.
Somewhere, there has to be validation phases. Just because you have objects, doesn't mean they are well formed.
https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-va...
It turns out proper validation is way harder than parsing. There is a reason text based interfaces and formats are so pervasive.