There isn't a mistake you've mentioned that we probably all haven't done too, so don't feel bad. I live in the burbs, so I ammended all my beds when we initially landscaped with adapteds and exotics that needed alot of TLC. Now that I've changed to WELL adapteds and natives, they got the benefit of ammended soil. I have clay too. You probably don't really need to ammend you soil - as your garden matures and you use leaves and mulch, and tend to your microherd and earthworm population, they will do that work for you over time. What I've done with areas that retain too much water or is poorly drained, is to plant things in those places that like, or can tolerate poorly drained soil. They are out there, you just have to research them and try them. When transplanting plants that are too close together, I usually wait until I see new growth in spring and dig as much of the root system as I can get and then replant. You'll need to water those regularly just as if you were planting new plants. Give them a soil drench at planting time with some liquid seaweed or Miracle Grow, or whatever you use as a liquid fertilizer. If they are a deciduous shrub, cut them back about a 3rd to compensate for lost roots.
Not knowing what plants you've planted I can't say when you should start seeing new growth. I've got things popping up now in DFW, but I also have some that I know won't start showing growth till April or so. I would think at least some of your perennials should start showing some new growth now.
Not ever grown a Texas Mountain Laurel, so can't help with that one.
Part of the fun of gardening is trying things to see what works, and remembering the failures so you don't repeat. Just make it fun and don't fret over the failures - just carry on.

Natives need no amending to the soil, in fact this could be detrimental. For transplanting, root prune this year and transplant early next spring before new growth starts. To root prune, take a shovel cut the roots all the way around the plant about a foot from the stem. Using a trench shovel is best because of its extra length but a regular shovel will do. When digging out the plant dig a moot like trench around the plant, NEVER dig into the roots but dig around, and then under cut the plant. Oh, make sure the soil in very moist before doing this so as to insure the root ball stays together as much as possible

Petzold is correct about root pruning, especially if the plants have been in their spot for along time. Since you just planted them last year their root systems probably haven't traveled much beyond the original root ball especially in clay, so if you need to move something newly planted last year, I think you'll be ok to do it now. I've done it and not had many problems.

Good advice so far. The only thing I'll add is about the Mtn. Laurel ~ is it in a spot that's holding water? If so, that's probably the cause of the yellowing leaves. It doesn't like too much water. It can tolerate it for a while, but eventually it'll show signs of distress. If you can, transplant it to one of the best draining spots you have.

Add to the transplanting the tree info: Even in well-drained soil, make sure that the hole is twice a big as your root ball, but not deeper than the ball. Dig and "ugly" hole with long furrows around the sides to allow for extra drainage. I probably would add some sandy loam to the clay soil, but you want the soil close to the roots to be able to hold some moisture without being soggy. Yellowed leaves are a sign that your tree may be drowning. Cheryl

Since Mt. Laurel is an evergreen I'm wondering if it's just doing it's "off with the really old" and those are the ones that are yellowing. I've not grown this tree, I'm just going off experience of some of my evergreens whose old leaves yellow somewhat before they fall off, but it's just a natural thing for them. Of course if it's in a soggy area, that is probably the problem, but if it isn't it might just be normal.

I'm farther south than you are, romandub, but transplanting in the winter (December-January) works best for me. The plants seem to bounce back well and they send out new growth in the spring right on their normal schedule.

I echo petzold on the idea of root pruning, but your plants are young enough that I don't think it's completely necessary.