Hi there! I'm joining in because I LOVE the excitement that comes when someone is planning their first trip

I also think it's wonderful that you're going with your mom, and that you get to travel so much with her. AND, I'm jealous you're staying at the GF because, after years of not being interested in staying there it has recently become a resort I really do want to experience. We tried renting points there for our Nov. trip but we waited too long. I've been there before for meals and it's absolutely gorgeous!
Now I'm no expert but I'll speak to some of your concerns as far as my opinion
1. Afternoons in Parks: yes, normally not the suggested time to get there, especially for 1st timers. However since there's really no way around it for you, you have it right that with your initial 3 FP's you'll get 3 of the more popular (or popular to you) rides in, and hopefully find good 4th, 5th, etc. FP availability for attractions you want. The other thing going for you with it being July is that many people go for RD and take a break in the afternoons. Not to make you think the parks will be empty by any means, but there will be a significant # of guests who leave when you're getting there. Also if you plan to stay late in the parks the crowds should go down somewhat so lines won't be crazy.
2. Heat. I HATE the heat

Like really, really, really despise it and normally don't go outside once it hits 80 degrees. We'd never gone to WDW during a hot month until Aug. 2014 due to oldest DS entering Jr. High. I was very nervous and afraid I'd be miserable, but ended up pretty pleasantly surprised at how well I handled it. It was freakin' hot, and sometimes downright uncomfortable, but that uncomfortable feeling didn't happen as often as I thought it would. We'd planned on taking a mid-day break every day which wasn't the norm for us; after day 2 we stopped that because we realized we could handle the whole day in the parks just by planning well.
3. Food. Yes, for many of us who go often food is a big part of our trips. There are so many great places to eat!! And like many others, my family loves to eat

On the other hand there are plenty of folks who only do QS, whether its because of price, because they don't want to waste too much park time sitting in restaurants, because they don't mind QS, or just rather spend their $$ on things more important to them than food. Since your trip is on the short side and you don't have full days for the parks, I don't think there's anything wrong with only having one ADR. What's more important is that you're planning on many grown-up slushy drinks
4. SW fireworks and dessert party. First of all, if you can manage to see them, see them! We saw them in January and they were fantastic. Granted the ones in January will be different but I guarantee they'll still be fabulous. As for the dessert party we haven't done one in years so I don't follow how far ahead of time they open the dates, sorry! Hopefully someone else will come along and answer that one.
Hope I helped just a bit! I'll be following along as you continue to plan!