work with long (actually very long) file/dir names.

[m^2]

Maybe I'm missing something obvious - but why not use dynamic strings (i.e. with a size allocated according to current content length) - especially if memory usage should be an issue?
Since most paths are short anyway (shorter than 260 characters), it would save memory almost always...

Because it's additional development effort. I'm 95% sure there's no other reason.

[Ammammata]

Maybe the best solution to store so many path/files is to create a database exactly like the directory structure on the disk (and consequently manage it in the proper way)
Maybe...

[petermad]

create a database exactly like the directory structure on the disk

Isn't that what treeinfo.wc is ?