Hi Nano,
I’m sorry to hear about your experiences with your Posh & Lavish mattress and it was certainly unfortunate to say the least. Based on your description it sounds like your mattress was defective (which is fairly rare but it can certainly happen on occasion) and it probably would have been a good idea to talk with Posh & Lavish to discuss a warranty claim if your retailer wasn’t willing to do so on your behalf.
They certainly use good quality and durable materials (Talalay latex) but of course different people can have very different needs and preferences and a mattress that would be “perfect” for one person may be completely unsuitable for someone else to sleep on that has a different body type, sleeping style, or personal preferences.
Comfort guarantees and refunds are offered by the retailer and not the manufacturer of a mattress (who only deals with warranty claims) and whether a company offers a refund or a comfort guarantee has little to nothing to do with their confidence in the quality of their product and much more to do with the price of the mattress and the individual needs and preferences of different people. Exchange and return policies are never free and are built into the cost of a mattress so the large majority of people that never exchange or return a mattress are the ones that pay for the minority of people who do (see post #25 here and post #3 here about exchange and return policies).
While nothing has a 100% success rate … with a local purchase and for the majority of people … careful testing using the testing guidelines and suggestions in step 4 of the tutorial (rather than just testing for the more subjective “comfort” or “showroom feel” of a mattress which often won’t predict how well you will sleep on a mattress or how it will “feel” when you sleep on it at home) along with some good guidance from a knowledgeable and experienced retailer or manufacturer that has your best interests in mind will usually result in a mattress choice that is well inside a suitable comfort/support range and will generally be “close enough” so that if any fine tuning is necessary it would be relatively minor and involve different mattress pads, sheets, mattress protectors, or perhaps even a topper if a mattress is too firm (see post #4 here and post #10 here).
For those that are making an online choice where they can’t test a mattress before a purchase or are closer to the “princess and the pea” end of the scale than the “I can sleep on anything end of the scale”, have a history of choosing mattresses that are unsuitable for them even if they have tested them locally, have body types or health considerations that are more challenging and that make choosing the most suitable mattress more difficult, or that for whatever reason are more uncertain about whether their choice is “right” for them … then the options that are available after a purchase to fine tune the comfort or support of the mattress or to exchange or return the mattress or individual layers may become a much more important part of their personal value equation but these options come with a cost (hidden or otherwise) and there are also some people that are more confident about their choice and may not need these types of options or wish to pay for them.
Return and exchange policies are part of risk management and how important a return or exchange policy may be for any specific person will depend on the person and their circumstances and on the level of risk they are comfortable with.
In the end … the level of risk that someone may be comfortable with in a mattress purchase is something that only each person can assess and decide for themselves.
Thanks for taking the time to share your experience … and I hope your next mattress purchase is much more successful 
Phoenix