I would definitely ease up on the watering - too much will quickly cause it to rot. Try to be patient and just let it die back somewhat this year, even all the way back to just a few leaves and the center spear; that's pretty normal when you transplant a Yucca. It will need all this year and probably next before it's really thriving again and sprouts another flower stalk.

I agree - that sounds like A LOT of water, even for a newly transplanted, large yucca. I'd cut back to at least only once a week, and then maybe once every couple of weeks or so. It probably won't need root stimulator any more either - I thought that was just a one time shot kind of product anyways...

If it survives to next year, it probably will need very little or no supplemental water once established.

The tree has received nearly daily deep watering and frequent doses of root stimulator.

STOP WATERING IT! You are drowning it.

The recommended way to transplant yuccas is to transplant them, and DO NOT WATER for a week or so. Then you soak it thoroughly and LET IT DRY (get a moistuyre meter and use it). Then soak it thoroughly again ... repeat for the first growing season. The second season, soak it thouroughly every month or so and it should be fine.