If the marigolds were 2 inches high or more and were touched by the herbicide, they would show obvious symptoms first, before they die, even this early in the season. Curling, deforming leaves, and it would begin 4-8 days in the cooler weather. They would die soon after (say 5-10 days). Frost damage would show itself the next day, or the next day it gets above freezing.
The smaller the marigold seedlings are, the more they are vulnerable. If they just had one or two sets of true leaves (the fern-like ones, not the first single long narrow ones), they could be killed in a day.
Powerzone contains the effective chemicals to kill broadleaf weeds like other lawn weedkillers, plus another that works better in cooler weather. I don't know about Speedzone. I am not saying that chemicals are necessarily good or bad for the lawn, but the company was smart to use Powerzone, IMO, as they could use less chemicals overall.
Everyone has their own level of comfort with herbicides, but you did a lot by having the forethought to cover your beds prior to spraying. Most would not have done it, and things are usually fine, I'd say for 75% of people. If it were me, I wouldn't worry about it, even with vegetables. Still, if you do find vegetables with abnormal, deformed curling or cupping in the leaves, I would discard the plants and all veggies that would have been similarly exposed. Not that it would kill you if you ate them or that you would even get sick, but you just don't know. So be safe. Tomatoes are very sensitive and are good indicators of contamination. For non-edible plants, if they don't die, they usually just grow out of it with time.
If you are worried about an area where you dumped some of the water from the sheet plastic into the beds, again, it is the chemical comfort level thing. For vegetables, it will take 3 years for possibly contaminated field to be labeled organic. (It may also depend on what chemical is there.) Myself, I wouldn't worry after a year for vegetables in your case. I wouldn't worry at all for non-edibles.
Thank you for being as thorough as you could with your explanation. I couldn't have been as extensive with my answer with out it.
