Hi jege41,
No foam does very well when it has been compressed for long periods of time in transit or storage. If it didn’t come right back I would call it “defective” yes … even if it wasn’t defective when it was made. This is more common with Dunlop that has been compressed for long periods of time but regardless I would send it back.
I don’t know the answer to this because it would depend on too many variables (including your body type, sleeping positions, perceptions, and preferences) and on the type and degree of change from your current mattress you are looking for that only you can answer. The most accurate way to know for sure would be to test a specific mattress/topper combination in person (either before or after a purchase).
If this isn’t possible (and of course in many cases it isn’t) … then it would be based on more subjective factors and the guidelines in post #8 here would be the guidelines I would use.
I would probably be tempted to exchange for another Costco topper to see if the next one is better. The worst case is that it would give you a reference point of how a 28 ILD 3" Talalay topper feels and performs in combination with your mattress with little risk attached 
Phoenix