How to make Alias to external nested folder?

Aliases, as a piece of Mac technology, don’t act as proxy folders as such (like symlinks do). An alias to a folder simply points to that folder as an individual thing. If you create an alias to a folder in Finder, and then drag the alias into the binder you’ll see why we don’t support selecting them. (And that by the way is a hint for how you can bulk import aliases a little more easily than using the menu command.)

That in and of itself probably is an insurmountable barrier, though as a concept it would probably no longer be an alias as a technical component but something else entirely invented for Scrivener. The larger problem with your idea is that it would have to effectively usurp the binder’s entire toolkit, where everything below that level would stop acting like the binder, and act like the file system. You wouldn’t be able to edit the organisation of it, add notes to that area, the outline-based principle of how the binder works would be broken (strict group vs file dichotomy), and items within this area would come and go based on the state of the disk, causing disruption to cross-reference interlinking and other issues.

Overall, it just wouldn’t be a good fit, and considering all of the limitations that would necessarily be imposed upon the listing of items within this “folder”, you wouldn’t get a whole lot out of having that you couldn’t just do with a Project Bookmark pointing to this folder, and a Finder window to the side of your project window. A trick to know about is that if you turn off the toolbar and sidebar in a Finder window it enables a kind of retro mode of how Finder used to work in the old days—where there was a tight relationship between a window and a folder rather than having this Microsoft Explorer file browser approach they went with for Mac OS X. Point being: the size and position of the folder, along with its view settings, will be persistent whenever you load that folder directly (not in a browser).

Alternatively, I could import all my research material into a separate Research Only Project, but is there a way to link an entire project within my main project?

That’s a pretty viable way of working in my opinion, and an approach I take myself with a few projects. I like to keep my working drafts lean and mean so that I can make frequent backups, and that means almost no media. Here is a description of the approach I take.