Hi javaqueen,
No … it doesn’t sound right at all. 17 - 23 would all be in a soft range. Savvy Rest doesn’t disclose the ILD of the layers they use but their approximate range is in post #2 here (or at least this is what they were at the time of the post but they would still be in a similar range).
This depends entirely on the type and quality of the materials inside the mattress. While all mattresses will go through an initial break in period where the foam will lose any of it’s “false firmness” and will soften slightly (see post #3 here) … if a mattress uses high quality and durable materials the you certainly won’t be sleeping in a dip in a year.
Latex is the most durable of the foam materials yes although softer versions of any material are less durable than firmer versions so it’s always important to make sure that you are making a suitable choice in terms of PPP for your body type, sleeping positions, and preferences regardless of the type of material in a mattress. There is more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
It sounds like you have been talking to some salespeople that are not particularly knowledgeable.
Phoenix