To me limp plants are due to 1 of 3 reasons.
Lack of water, too much water, or root damage which results in lack of water.
I was wondering why you added lime. I have never thought to add that to potting soil. Were you worried about the PH? I am not saying it is wrong, just I wonder why. I looked it up to see if it can burn the roots of a plant and it can. Root damage can prevent the roots from taking up water and results in wilt. http://homeguides.sfgate.com/can-much-lime-burn-plants-83263.html
What comes to my mind first when I see a wilted plant is lack of water which is the easiest to fix!!! You say the soil was dry except the very bottom was a little bit wet. Was the rest bone dry? If so I would sit your plant in a bucket of water for 10 minutes then drain. Lavender hates being too wet, so make sure you pull it out of the bucket soon. Is it very hot or very windy in Tustin, CA? These conditions tend to dry out pots very quickly.
The trouble when you repot a plant, especially one that is root bound, is the new soil sits on the side of the root ball and the water can pass thru that new soil with little resistance, going around the root ball. That is why it is suggested you push down on the new soil to compact it around the root ball. The water passes thru fast but the rootball gets none of it and stays dry. Sitting the whole pot in water, allows the water to soak into the root ball. Only you know how root bound your plant was.
My next thought would be too much water which might be the case with Lavender which hates too much water. This can happen when you water too much, never letting the soil dry out so no air gets to the roots. They drown. The roots rot and then they cannot take up any water and the plant droops. How did the roots look when you took the bottom rocks out? Were they healthy with white looking feeder roots? Or mushy?
It does not sound like you over watered since you only watered once, so next look to your soil. It can also happen when you over pot your plant and put it in a too big container with too much new vsoil that stays too wet next to the root ball. Again you get root rot. I looked up the soil you used Miracle-Gro® Moisture Control® Potting Mix and see it holds 3X the normal amount of water. Lavender hates wet roots, prone to root rot. Interesting you mixed that with cactus mix. Maybe that reduced its water retention ability. https://www.miraclegro.com/en-us/products/soils-amendments/miracle-gro-moisture-control-potting-mix
The Miracle Grow soil you used seems to contains fertilizer. Fertilizer can burn the roots of a plant. I have done this more than a few times myself.
If your roots look bad, I would cut them off till you get to healthy roots. Repot it in new dirt in a pot only a bit bigger than the root ball. But then you have to cut the upper part of the plant to correspond with how much of the roots you took off. For a smaller root ball may not be able to support a big upper plant.
But now you have cut off all the blooms. I do not know if your lavender reblooms quickly. I would think that you only wanted this plant for the blooms. Is it worth it to keep maintaining it, if you do save it, for a year till it blooms again? Most plants I would buy from Costco like this, I treat as annuals just to enjoy the bloom. Lots of them are treated to over produce their blooms, which exhausts the plant and that makes it a poor candidate to plant it in the ground if that is what you intended after the show.
I hope I have not made it sound overwhelmingly complicated and discouraging. I might be after sooo discouraged reading what I wrote. TMI LOL
Hopefully, it is just lack of water which is so easily fixed. Potted plants are known to dry out quickly. You will find you learn quickly the more you venture out to garden. It is so much fun and can be so rewarding when things go right. If this one did not work out, try try again!
http://homeguides.sfgate.com/growing-french-lavender-44128.html
GOOD LUCK!!!!!
