Hi Sunshinegirl,
I think with your lower weight there would be less risk of alignment issues with the Beautiful than if you were heavier and in your circumstances I would probably put more emphasis on pressure relief than would be normal (normally alignment is the most important function of a mattress because it allows the discs in the spine to decompress and re-hydrate and the muscles and ligaments to relax and recover). I would also make sure that you do good objective testing (along the lines of the testing guidelines in post #1 here) if you go into the store to re-test mattresses rather than going by “feel” alone which can be deceptive although your own sleeping experience will be even more accurate than testing in a store and you have some time to make sure the mattress is working for you.
Latex like any foam will soften over time and even develop some slight impressions … it just does it over a longer period of time and to a lesser degree than other types of foam. A mattress will also go through a more rapid softening period over the first 30 - 90 days or so as the foam loses any of it’s initial stiffness and covers stretch somewhat. You can read more about the factors involved in durability in post #4 here and the posts it links to.
I’ll be interested to see what your detective work uncovers about why your mattress may feel firmer than the one in the store and what the cause ends up being.
Phoenix