I took a look at the images you provided. I do not see anything wrong with your seedlings. If they are going mushy the photos do not show it. When I plant my seeds I try to use sterilized soil/sand. I bake it for over an hour at 225 F. I also like to keep the seeds/soil in a container with an airtight lid (transparent 6" wide by 3" deep take-out containers are good). I like to keep my soil just slightly humid. It should not be wet to the touch. Every few days I check to make sure the soil has not dried out. If it has I spray a few spritzes of water to moisten the surface. The soil is in a sealed container so the moisture will eventually fill more or less evenly. I keep my seedlings under a daylight fluorescent bulb. I think it is 20 watts or so. I use 4 bulbs and have the seedlings about 10-12" from the bulb. I leave them in the sealed container for at least 6 months, sometimes a year. When they are large enough I take them out and plant them into SMALL pots. Something Like an old styrofoam egg container is good. Poke holes in the bottom of each well so it will not hold water. You can cut off the lid and use it as a base. It makes for a sturdier way to pick up the set of 12 wells. Allow the soil to dry out between watering. At this stage you can move the plants to a bright window. Be careful to introduce the plants to strong sunlight (U.V.) gradually or the will burn (usually they go red when they burn). That's how I do it.

Hi Kitkat & Welcome, I'm going to assume you don't know about these plants, so if I say something which sounds obvious or that you already know, I ask in advance that you pls. excuse me. Xmas cacti are jungle cacti, not desert cacti (as some folks mistake them), so they want & need water. They naturally grow in the branches of trees in the tropics, so in these branches or crotches of trees as they're called, they don't really have soil, just leaves, debris & animal droppings that collect there. Key is that any water runs right through this debris & doesn't stay in the roots long at all. The roots of these plants can look surprisingly small & indignificant. What do you mean by the soil looks like cinder? I don't know that word, does that mean like burnt ashes of firewood as example, like burnt cinders? I agree with statement above that woody growth can be normal, just a sign of older growth, so pls. disregard that. I suggest you take cuttings & start new ones; then repot the old, original plant as well. The leaves are referred to as segments or stems. It's recommended that a cutting should have 3-5 segments on each one cutting. I'd use small plastic pots, & place a number of cuttings around the outer edge of the pot & one or 2 in the center. I just checked my reference book & confirmed that largish plants can be kept in smallish pots, say a plant 12" wide & maybe 8-10" high, could easily remain in a pot only 4-5" in diameter. The mix should be something like African violet soil readily available at nurseries, box stores, etc. w/ added perlite, say 1/3 perlite to 2/3 AV soil. This will make for light & airy mix, fast draining which is what these plants need. Only water the new cuttings lightly as without roots, they can't really take up the water. Give them bright, indirect light or place them at the edge of a window. Either water lightly every few days or spray the top of the mix w/ a plant mister. Pls. don't tug on the cuttings to check for rooting, just wait to see new growth; when you do that means the cuttings have rooted; then you can you can water more. If the original rootball & its mix are hardened, soak them first, in a basin of warm water & crumble off the old mix w/ your fingers as much as you can & repot in a fresh mix. Hope I've covered everything, you should see new growth in 4 weeks I'm guessing, if not before then. Good for you for wanting to rescue & do right by this plant. May it reward you w/ lots of blooms next year. Are the edges of the stems jagged or rounded? Jagged indicates Thanksgiving cactus, rounded indicates Christmas cactus. Good luck!

socks, mine is not unmanageable it's just beautiful even at its size. It is an outside plant year-round on our 15 acres property, compared to other plants and trees we have it not so big in our eyes...

The plant has been in the kitchen window, above the sink, which actually forms a corner, so there's one north-facing window and one west-facing window that come together. The net effect is that the plant gets A LOT of sun every day, albeit behind glass and screens. Once it's outside it'll have to deal with much crazier heat than it's used to indoors, especially if I locate it where it gets as much sun as it's used to--but I think that's probably not the smartest idea. I can keep it sheltered somewhat as noted earlier, i.e., on the covered, shaded patio, but then it won't be getting much sun...which it's definitely used to. You know, I think I might be changing my mind again. I seem to do that a lot after posting here! How does this sound instead of moving it outdoors: put it in a larger pot INDOORS! Keep it in the kitchen window where it's done so well, but just give it a bigger 'house' so to speak.