Chickweed, Stellaria media, a cool weather annual weed. Very easily pulled up but do it before it goes to seed!! Also edible if you care to get rid of it that way :-)

I agree, it is Golden oak. It will take several coats to get the color of your existing stain, because, it does change colors as it ages. It will also appear a bit darker when you add poly for the final finish.

By now you may have gotten an answer to your query, but if not the Fl Dept of Agriculture, County Extension Service may be most helpful. I have contacted them in the past with good results. Every county has an office, so just look up the dept of agriculture and find your county. The contact information will pop up. You can then submit your photo and contact info. I'm sure they will be able to identify your mystery plant.

You will no doubt get some conflicting opinions about this but IME, terrariums are NOT a great growing environment for any succs or cacti. These are best grown in a very fast, mineral based soil mix with excellent drainage. Terrariums offer none :-( And mixed plantings are also less inclined to produce healthy results from the plants as few will need the exact same conditions as their partners. If you want to grow cacti and succs successfully indoors, one plant per pot and make sure that pot offers adequate drainage opportunities.