Can all or any smartphone manufacturer ship their phones with the stock, unmodified version of Android besides Google? Can other smartphone manufacturers beyond google sell phones with stock android?
Depends on what you mean by "stock Android". Android itself is open source (see Wikipedia: Android Open Source Project (AOSP)). But that doesn't include the Google Apps. If they want to ship those, they need to sign the Mobile Application Distribution Agreement (MADA), which comes with a set of rules (which Google apps they must install, which must be placed prominently on the home screen, and more). It also requires them to get their devices "certified" by Google.

If they are willing to take that burden, they of course can. Many manufacturers still chose to ship devices with their own modifications of the user interface, to make their products "more outstanding", and also include a bunch of bloatware of doubtful value (often duplicating what the Google apps forced upon them by MADA terms already provide).