Obviously is [sic] a grid has two numbers missing from the clues, there'll always be at least two solutions.

Note the 'Obviously' and 'always'...followed later by:

As to whether you can get a puzzle with two numbers not present in the clues, but with only one possibility for which 9 of the 18 cells one of the numbers goes in, I think that's unresolved

I'm sure you've just disagreed with your first sentence there Paul.

I'd imagine it isn't true, myself. Essentially, you are conjecturing that a puzzle can be given that is missing two numbers from its original clue set but somewhere along the way it becomes apparent that, in fact only x can go here and y here.

I don't understand how you arrived at that conjecture. (I suspect it's just a gut feeling and I respect that, but I'm just being challenging.)