Roundup actually kills the plant by attacking the roots, so, yes, is the answer. When you Spray the leaves, (best after a little rain) the plant takes the liquid down to the root and it kills the whole plant. Tough weeds might need more than one application.
No point in tilling/rotavating until Roundup has had a chance to work. Read the container for methods and strengths of application and try not to spray on a windy day!
My plot was mainly used for veg but, yes, I did grow common blooms to attract insect pollinaters. Flowers and veg make great companions....might have a pic or two around.
Yes, I added quite a lot of compost to the ground to try and lighten it a little. Heavy clay soil is very fertile but a real pain to work even after just a light shower, so adding compost/mulch/manure is the way to deal with it. Even gritty sand helps break down the caly sods.
Couch grass roots!