Slim - 50* is too cold for your plant. Both photosynthesis and water usage slow to a near stop at temperatures that low, so it sounds like you're almost assuredly over-watering. Adding potting soil and fertilize are likely to have provided the impetus for the death knell. Covering the crown just improves the living conditions for the organisms that have already been at work rotting the stems, and the added fertilizer may well have simply added to a high level of salts probably already in the soil. Most houseplants do best at temperatures above 60*, and 55* is about the lower limit for good growth/vitality for a variety of reasons. Most houseplants like a soil that drains freely - so freely that if you water to the point of saturation, with a good amount of the water applied moving through the soil and flushing accumulating salts out, the soil drains well enough that you don't need to be concerned about root rot. We can talk about fertilization, watering technique, soil choice, light, and other important considerations, but I'm afraid this particular plant is probably in serious jeopardy. Al

Judy I had Silver Glade Trinkets which had very grey/silver leaves, no mottling. I also had S.G. Moons which had normal green leaves but it never flowered before it expired. The flowers on S.G.Trinkets were reminiscent of Optimara Cora but much prettier. It bloomed constantly for about a year until it expired. The silver leaves may be a feature of the Silver Glades although the photos don't show that particularly.

I would defer to someone in your immediate vicinity as Long Beach may have some unique weather conditions for specific advice. Generally speaking you can transplant a plumeria in ground after the threat of frost has passed. The preferred soil can be sandy, loamy or even rocky. I would amend any kind of gumbo, silty or heavy clay soils to improve drainage. Essentially you do not want an in ground plant standing in water for long periods of time. Stake it to keep it stable and then fertilize from April to October (perhaps longer for CA). That should be enough to get you started.

Schefflera arboricola, Begonias (cane, wax, rhizomatous - like Rexes and Beefsteak,) various Ficus, Dracaenas, parlor palm, snake plants (Sansevieria,) really any but those that need full sun (6+ hours of direct light,) and those at the far other end of the spectrum that don't appreciate any direct sun at all. What kind of Kalanchoe has caught your attention?