If your ute has original paint & it's obvious that there have never been any holes for a mirror, then you can argue that, in your view, it's plain that the vehicle was registered from new without a mirror on the passenger side.
If it was OK, back then, & for every year since then, why would there be a problem now.
If there is evidence that a mirror has been fitted before, then there's a problem, ..... however, the hot rod blokes often use a peep mirror that is clamped to the edge of the door. These are often used on 1933 & '34 models because they were usually rear hinged [suicide] & this is a practical means of providing a mirror.

No reason why it wouldn't work on any other type of door & could be easily removed or re-fitted as needed without the need to drill holes.

As for the heater, what happened to it? As far as I can tell, they all came with a heater/demister in Aussie spec vehicles, so I assume it had one at one time.