Should (Advice / Expectation)

Suggesting a course of action to a customer, a guest, or someone senior.

～たら いかがですか vs ～たらいい: Use たらいかがですか. It is the polite suggestion form and keeps the decision visibly theirs. たらいい, といい and ばいい are all casual and near-interchangeable with each other, but any of them to a superior reads as telling them what to do.

Saying something 'should' be so: is it advice, or a prediction?

～はずだ vs ～ことだ: はずだ is expectation from reasoning ('it should be true, given what I know'), and says nothing about what anyone ought to do. ことだ is direct advice ('what you should do is...'). English 'should' covers both, which is exactly why they get swapped.