This is a perfect exemplar of something I really hate about this website. A poster asks for advice about how to buy a diamond, and instead he gets mostly replies saying "don't buy a diamond." I will try and actually be helpful.

Being pedantic, the original question was

I would like to propose to my girlfriend in the near future. For this I would like to use a diamond ring.

and your first suggestion was

Propose with a "fake" ring, then go shopping for the "real" ring together.

This seems like a reasonable suggestion. But I think you applied the same heuristics as others, just less far. Those heuristics being "infer motives from question; give advice satisfying inferred motives". The motive you inferred seems to have been "I'd like to propose, and I'd like my girlfriend to end up with a diamond ring". Others seem to have inferred something closer to "I'd like to propose with a ring with a pretty stone in it".

Basically I think that "being helpful" is a difficult game, and "answer the question as asked" doesn't lead to optimal helpfulness, and I don't have a good solution for this.