I have been asked on more than one occasion if it was possible for the family to have gold crowns and fillings returned to them prior to cremation. I do know that my embalming and morgue staff will refuse to do this since it will require extraction of the teeth with pliers and as such is rather destructive to the remains. I have heard of other funeral homes that have accommodated this request however required that a dentist be hired to do this at the expense of the family, however none of the families whom I have worked with persisted on this request to this degree and most simply give up on the idea once they realize just how intrusive the process will be.
Gold and any non combustible items - such as surgical implants, hardware from the casket, detritus left over from any mementos that the family placed in the casket for cremation - are removed from the bones before the remains are processed to an aggregate substance resembling “ashes” - as to what happens to these items I do know that my local crematory disposes of these items, and I have heard of other crematories donating the metal to a charity for recycling. It would be unethical for these items to be harvested and sold for profit.
Most remains encased in a casket these days are embalmed. Additionally, metal caskets are constructed with a gasket that makes the casket airtight - this can pose a potential problem as the remains inevitably decays in the airtight casket. Even after embalming there is a multitude of bacteria that will still cause the remains to decompose in the casket, often releasing gases that have in the past caused airtight metal caskets to buckle from the pressure and in some instances actually burst from the pressure. In this regard, the sweetish odor you detect in a mausoleum may be a combination of rotting necrotized flesh, embalming fluid, deodorizing compounds and cleaning fluid.
All burial vaults, regardless of how well constructed to be moisture resistant will inevitably break down to the point where moisture will breach the vault itself. In the case of a vault containing a casket, it is entirely possible that the moisture will eventually breach the casket itself and reach the remains. I have personally done disinterments where the remains inside of a casket were completely submerged in fluid and had decomposed in the fluid to the point where the only thing to reinter was a few bones fished out of the casket soup along with articles of clothing and hair. Conversely I have done disinterrments where the remains were bone dry and in good enough condition to be recosmetized and viewed by the family even after 25 years. I have also done disinterrments where the vault had been breached by water in which the casket itself was floating but otherwise uncompromised. So there are various factors at play that can cause the remains to become floaters, however this is not the standard case scenario for every burial.