Well I brought it home last night. I wanted to bring it home on a Thursday since I work from home on Fridays and hence can monitor things.
What a night last night.
I put the anemone on the isolated rock pile towards the front right of the tank, where the purple gorg and pink birdsnest used to be (relocated them of course). I even built a little eggcrate cage around the top of the rock to keep it isolated from fish while it acclimates, and ideally to keep it from blowing around the tank.
Well, it partially worked. Fish aren't bothering it, but it's not exactly grabbing on to the rock with a solid grasp. I went to bed around 10 last night and it was partially attached to the top of the rock pile. I went down around 1 to check on it, and it had somehow managed to squirm through a TINY hole (compared to the anemone's size) in the rock, into the main cave in the middle of that bit of rockwork. It's foot was barely attached. Not wanting to disturb it I left it and tried to get back to sleep, but instead laid in bed until about 4:30 worrying about it.
All this time my spidey senses were tingling like there was something wrong, so I went back to check on the tank and noticed the overflow level was way off, as if the return pump's flow had been cut way down. I shut the pump off, and in the backwash as the plumbing drained down, my emerald crab washed back out of the pump.

Poor guy probably died from dizziness. He was intact save for the end of one leg but definitely deceased. Not totally sure how he got into the pump in the first place, as he would have had to get past eggcrate on the drain, then eggcrate in the sump separating the return chamber.
Anyways, around breakfast time this morning, the anemone had let go of the rock and plopped down to the sand. So I scooped him back up and put him back in his eggcrate cage on top of the pile, after blocking off his last escape route with a bit of rock rubble.
He's not looking terrible - mouth is tight, most tentacles are inflated. But he's definitely still not happy. Much smaller than he had been in the store and generally looking ****ed off. I can't tell if he's grabbed on to the rock or not. Time will tell. Fingers are crossed...