The only way to kill Bermuda grass short of digging out the yard is Methal Bromide, which is not available to homeowners and will be off the market in a year. You can try all you want with round-up it's going to come back. As far as the grubs there is a certain time to apply depending what kind of insect you have. If you see them as adult form then its too late, you missed their cycle. Check out some common grub damage and compare to yours and see how best to attack from there. Most grubs have a certain stage that they are susceptable to pesticides and it must be timed just right.

Some of us love it, and some of us hate it! I have been using a product (sold at most stores with a comprehensive lawn and garden inventory) that has been successful in pushing back and keeping my neighbor's Bermuda grass off of my lawn. He is among the group that thinks if it is green it is good. The product does not claim to be able to kill Bermuda grass but it does effectively control it. It does claim to kill crab grass but I do not have a problem with crab grass so I cannot confirm this claim. The product is by Bayer, and comes in a blue hose-end spray bottle. The photo I attached was taken this spring before the Bermuda grass became active. I

Turflon Ester is just triclopyr, so you could substitute for Bayer brush killer, Weed-B Gon CCO or any other triclopyr herbicide, and adjust based on percentage of AI. Might be cheaper. I sprayed fluazifop+triclopyr for Bermuda control last year and it did kill the Bermuda, but took the fescue out with it as I was spraying in high heat. Let me see if I can find the rate I used when I get home this evening. I think Zoysia is actually supposed to take the Ornamec better than fescue.

Ok, I get it. You don't want to name the place where you live. I used to spend a lot of time in El Monte, so I'm going to infer that you live close by there. I would strongly suggest you eliminate the Marathon completely. It is a water hog that far inland in the summer heat. I'm convinced that the mass marketing of Marathon I, II, and III is one of the big contributors to the current water shortage. If you lived west of I-5 then you might have a chance with it. West of the 405, certainly. But out in the area(ish) where you are it is too warm for a cool season grass. You can make it work, but the cost in water is too high for the state. Please consider doing your part to ending the water shortages. Alternative turf grasses for your area are bermuda, which you are familiar with, St Augustine, a coarse bladed grass that tolerates heat very well and is easy to care for, or some of the zoysias. If you have full sun, then an excellent choice would be UC Verde buffalo grass. This is a variant of the prairie grass that needs very little water to thrive. It may be green all year in your area depending on your altitude. Another benefit over other buffalo grasses is the UC Verde is very dense and never gets seed heads. The density fights off weeds and also means it never needs to be reseeded. All the warm season grasses are creepers so you have to keep them out of the flower beds somehow. But you chose where to live. You might be able to see some turf grass alternatives in action at the Huntington Library or Descanso Gardens.