You aren't supposed to cut grass more than 1/3 of it's current length. You have cut all the green off the top and only the woody part of the grass is left. Fertilizing will certainly help and cut it frequently to keep up with the growth. You can actually cut Mohawk as low as 1/4" but the lower you cut it the more you need to keep up with mowing.
The good thing about Bermuda grass is you can cut it lower than some of the other common lawn species and that helps control weeds. Cutting it low encourages it to grow out instead of up.
Just fertilize, keep cutting it low and it will come back.

Well what you experienced is what is suppose to happen, you removed too much at one time. But no real harm done.

What has happened is your Bermuda was trained by you to grow tall, rather than lay down. Now you have to train it to grow at a lower height. By doing this you will cause the Bermuda to grow much thicker and denser which will help choke out the weeds.

But there is a catch. You can maintain your Bermuda down to a 1/2 inch if you wish and have a reel mower to cut that low. Bu there is the catch to go that low requires mowing just about every day or at most every other day. You cannot remove more than 1/3 of the height. Maintained at 1/2 inch requires mowing again at 3/4 inch. Bermuda can easily grow 1/4 inch per day.

If you have a rotary mower, most likely the lowest you can go is about 1-1/4 to 1-1/2 inches which is just fine for an improved common type you have. Just keep in mind it will require mowing twice a week at that height.

So pick a target height, read the Bermuda Bible, and follow the instructions. Just be aware the height will determine mowing frequency.

Don't worry about the cool season grass, it will be toast if you water, feed, and mow Bermuda properly. Fescue will roll over and dies when cur short, heavily fertilized in the heat of summer with with only 1 inch of water per week.

Thank you both for the responses. I actually knew about the "1/3 rule" but I had to cut it lower sometime, so...

There's about 30,000 square feet of lawn here with the mixed varieties described above, some of it under high filtered shade from tall pine trees and I'm wondering if some of the newer seeded varieties like Yukon or Princess might be more appropriate in the slightly shaded areas? Is there a way to plant (overseed) the newer varieties so that they might overtake the Sahara/Mohawk without starting over from scratch?

This lawn is in the metro Atlanta area and the soil here is mostly Georgia red clay. I understand the red color comes from iron content and since iron is a recommended nutrient, can I release some of the iron from the clay by applying lime to raise the pH, which is typically around 5.5 in this area? Will applying lime in this sweltering heat damage the bermuda?

Or should I apply a fertilizer with iron in it at the appropriate time?

Bermuda in shade will thin out over time. How fast depends on how much shade. Also now that you cut the bermuda at 1.5" you will need to raise your mowing height or you will cut all the green off. If you want to cut at 1.5 you will first need to cut it down below 1.5.

Princess is very nice but can not stand shade. For Bermuda in shady areas, a few of us have gone to TifGrand (or Celebration...but it's not readily available in GA). After replacing a section of my 419 under trees, 3 times, I finally put down 2 pallets of the TifGrand. It is really great stuff. Can handle 60% shade, but grows like crazy in the sun. Requires less water and nitrogen than 419. But, it is about $170 a pallet from Super Sod in ATL. That's why I only used it in the shade.

You can overseed other areas. I did that in my back yard with Triangle. You do need to fix the pH, but it takes time.

I do use iron a couple of times a year. TW recommends the Bonide liquid product and it works great. Pike caries it.
But I think the pH comes first...

In reference to Bermuda and shade, they do not mix. Bermuda needs full sun. As mentioned there are two commercial varieties out that tolerate some shade, but that is 2 out of a hundred or so Bermuda varieties.

As for iron chlorosis and soil PH, it is high PH soils (7.0 and higher) and presence of lime that locks up iron and prevent iron uptake. You said you have a soil PH of 5.5 (acidic) with heavy red Georgia clay. While your PH is a huge life long loosing battle of a problem. it is the heavy clay and soil compaction that will cause iron chlorosis. It prevent oxygen (air) penetration into the soil. The liquid iron products help because the iron is absorbed via leaf surface, but is only a band aid and short term solution. The real solution is aerating and loosing up the soil so it can breath.