Here is the un-damaged corner.

Hi Marita40! I live in Atlanta, GA and while we don't get anywhere near the snow and ice that y'all get, we had a serious winter storm last week. Once I got the buildup out of the garage, I ended throwing down some gravel (needed more traction than sand and the salt would mess up the finish on the floor). Plus it helped when everything refroze overnight. Stay warm!

Get a disaster mitigation team in there pronto! The water damage may have spread much farther what is currently showing! You need someone to take moisture readings on walls and floors well beyond the obvious damage to make sure it's not a lot more extensive. The drywall has to come down. There is probably soaked insulation above it that will continue to hold water until it is removed and the internal structure is dried out. This isn't a one-fan job.

I'm glad you're up for this. You won't have any problems. When you mix up the concrete, the temptation is always to add too much water because it makes the mixing easier. Use the proportions given on the bag, and add water slowly. If it gets too moist, just add a little more dry mix. Below is a link to a 30 second video that shows you what you will be aiming for. Even though you will be using sand mix, and not regular concrete, you are still aiming for the consistency shown in the video. Also, you don't need much volume of concrete so start with a small batch. When the consistency is right, go around and force it into the cracks, leveling off with the trowel. Don't worry about it running under the foundation or putting pressure on it: the odds of that happening are slightly less than me getting elected as next Pope. Here is a link that might be useful: Concrete working consistency