Is substituting something non-alcoholic for the wine a deal-breaker for this recipe? If not, any suggestions for a substitution? Thanks!

Not a deal-breaker! Other commenters on the recipe have subbed in more broth (or a combination of broth and water) with a splash of lemon juice or vinegar for acidity.

Not sure what deal-breaker definition is for you.
If there were only a little wine in a recipe, leaving it out or replacing it would not make much difference.
Here, the cooking in wine-based liquid is a major contributor to the dish.
Yes you can replace the wine, but/and that will make a different dish, with 1 of these:
•white grape or peach juice, laced with a tablespoon of lemon juice or cider vinegar.
• pomegranate juice, which will also change the color.
• broth, which gives a salty, savory taste.
• water, gives moisture only.
If these options aren't to taste look for other roast cauliflower recipes without wine.