The floor was scarified but some glue is left. My contractor said he was going to finish the prep as he goes. Scraping old patches of glue that remain. Messy, dusty, with an adhesive install. Anyone else proceed like this?

Prepping a slab can be dusty

Yep. Why finish the prep as you go? All the dust is hard enough to manage and get rid of without the adhesive flooring process going on at the same time. This is prefinished. I think it sounds like a disaster. Do you know people who do this?

They make dustless grinder covers these days. Minimal adhesive residue around the perimeter isin't that big of a deal, it's out in the field is where is needs to be completely taken down. Grind high spots while your there.