I have more to add to my initial question. The tile installer is soaking the concrete with acetone and using what appears to be a buffer in an attempt to remove the adhesive and pad remnants. Could this be making it worse? I learned from the previous wood installer that the glue used was Dominex but I don't know if it is solvent or water based and can't find out online. I am so skittish about so many materials and processes and how they all interact. What if this installer can't do the job? Am I entitled to my deposit back? If there are consequences down the road, what are they likely to be and when will I know? These questions also apply to the adjacent room where a carpeting installer ground off the residual glue and rolled ardex quick moisture sealer in preparation for a carpet. Oh how I regret ever doing that glue-down engineered wood on a concrete slab. To anyone else considering it, tread very very carefully.

Here is the Ditra handbook, look at page 10 for info on concrete prep - it recommends removing oily or waxy surfaces from the concrete, or scarifying in if you have to (scratching it up). You can also call Ditra 1 800 472 4588 and ask your questions for answers from the horse's mouth. A photo posted here might help others tell you if it is ground off enough for the tile install to go smoothly. I am a DIYer and used Ditra as an uncoupling membrane on a slab, after scraping up old glued-down linoleum. There was a lot of black glue still down on the slab, but it was completely scraped flat and scratched enough to give some "grip" to it. That was in 2011, no problems thus far. You would have to talk to your installer about their warranty.

Here is a link that might be useful: ditra handbook