Prolly not. Do you know what kind of grass? I have Bermuda, but fortunately it is a not-too mutant strain and does die with Roundup. You may have created a paradox of sorts--the landscape fabric of course can't really kill the grass, but protects it from your Roundup. So all the healthy roots are just there waiting to poke up here and there. Either you will have to be super aggressive with the Roundup and spray each twig as it appears, every day for weeks, and even then you may have lots going on underneath and just waiting for the right moment to grow, so you may have to re-prepare the entire bed. You're hoping, I guess, you can smother it. There aren't any real smothering products for sale for your purpose. Depending on what you've got planted, you might be able to remove (and save) the mulch, remove the fabric, Roundup what you can see, place thick layers of cardboard or newspaper over the grassy areas, and mulch over that, and hope it is not the super-aggressive Bermuda or something that grows through wood! So you can read and search about smothering grass, lasagna gardening, no-till gardening and lots of related things. People will tell you things will work or not work, but it all depends on what your actual garden area, plants, grass and so forth are like. So, I have no idea whether you've got a grass disaster or something salvagable. And it depends on whether you just missed a few roots or whether you just put your stuff over living sod of an aggressive grass. How did you actually prepare the bed?

1. Most of the "weeds" that are growing in that mulched area may well be growing in the mulch, something that happens when the material is not periodically renewed, and can easily be pulled out, or killed simply be stirring the mulch material with a cultivator. However the "landscape fabric" can be removed, if desired, simply by carefully lifting it up and out and then replacing the mulch that was there. 2. You can aid in "weed" suppression, after removing the fabric, by laying down newspaper and covering that with a mulch material. That will also help suppress "weed" growth in the future, but there is nothing that will prevent "weeds" from growing if they have half a chance. Since Ma Nature does not like here soil left bare she grows what we call "weeds" to try to protect that soil fromt he ravages of the sun, wind, and rain. When we put down a mulch, thick enough to protect the soil, then those "weeds" will not grow. 3. You can work at digging out those "weeds" or you can lay down newspaper over them and cover that newspaper with a good mulch material and that can help suppress those "weeds". Be carefull with boiling water because that can also harm your wanted plants. Never, ever apologize for asking a question because that is how you learn.

I have a strong prejudice against landscape fabric. Yes, I say "prejudice" because it gives me a visceral reaction though it may be benign in many instances. Just the idea of all that polymer in the soil in my yard... That said- it will limit root depth. If you've had "outrageous success" with it at 6"- then it may be no big issue. I would anticipate that giving roots more room to grow could only enhance the garden, though many report success growing in shallow beds on hard surfaces. At 6" down- it really shouldn't be that much work. If you decide to take it out later- after you've added another 6" depth- now you're talking about some work.

I can't tell what the existing brick is doing--do you want to install rocks there? I can't help with that. But on the boxwoods--I thinks it would look nice to continue them under the window to the right. Remember that there are several different species and cultivars of boxwoods. Take a piece from yours with you to see if you can find matching ones. Do you know what the deciduous shrubs under the windows are? Just in case they are worth keeping? If you can continue the boxwoods and put your burning bushes in front of them, then I would want a low evergreen border. I agree on taking out the junipers. Where are you? location and zone will help with suggestions.