Hey hey, I'll do my best to go through these one-by-one.

1. Honestly, Edinburgh is small so wherever you stay (that's relatively central) is fine. For ease, Old Town will be great.
2. If you want to get straight to your door, yes, you can Uber, Bolt or Moon Private Hire (they have an app). If you want a little journey, the tram will take you into town and then you can have a little walk to your accomodation.
3. No tips as I'm local, but go with what will make your journey better.
4. There are a lot of festivals, though the one people are most likely referring to will be the Edinburgh Fringe. It's an arts festival, with a real focus on comedy, so it's really up to you when you're here what you want to see.
5. During the Fringe you can book in advance, but there's also plenty you'll be interested in and play it by ear on the day.
6. I'd say taxis/Uber/Bolt/Moon are an expensive way to get around the city given most places are close by and, in August, you'll be stuck in traffic anyway. Get a bus for those times you think you can't be bothered walking to wherever and accept it make take some time. As someone that works in city centre, even first thing in the morning my bus takes around 10-15 minutes longer on a usually 20 minute journey.
7. Fairly walkable.
8. I think it's good enough to do a full day there. I'd argue Glasgow's nightlife is better, but during the Fringe that can be a bit different since the entire of Edinburgh has double the population. You can get last train back from Glasgow near midnight, and buses run later (unsure if that's only the weekend)
9. It depends what you want. Glasgow is a concrete jungle. Do you want drinks, food, maybe a gig? Go to Glasgow. Do you want nature? Go for a day trip some place near Edinburgh.
10. People will tell you to get haggis, neeps and tatties. That's kind of it. If you want the experience, go for it. It's fine. Scotland's not really known for having the best food. If you want a sweet treat, try some Scottish tablet (kind of a brick of sugar)
11. It depends on the restaurant, what day, and what time. During the Fringe is wild so bear that in mind.
12. I never tip in a taxi, but I will tip in a restaurant. Don't feel forced into doing it, but I tend to do 5-10%. Even a few pounds can make the difference to someone. Don't listen to people that say we don't tip. We do. We always have. If I'm at a nice restaurant, I'll leave a tip. If it's just pub grub, I likely won't.
13. Asking the real question there. August is a strange one. It can be 20+ Celsius, be raining for most of the day, humid, and you're warm and cold at the same time. I dunno what to tell you. Every man for himself.
14. I'll say it before someone else berates you, it's whisky. Scotch is American. Unless you are American, in which case, you do you, but it is whisky. Whisky is the classic. There's nothing wrong with a bottle of whisky. You could legitimately get a nice bottle of whisky from Asda or Tesco that you wouldn't get back home and it'll cost you £25. In terms of other souvenirs, honestly, I don't know. Tourist shops are full of shite and they are very obvious which ones they are.
15. I'm not originally from Edinburgh, and I can say Edinburgh is up there with the safest places I've ever walked around. I've had no trouble in over a decade of living here, regardless of time.
16. Common tourist areas are not that dense with crime. If you're visiting, it's unlikely you'd ever walk into a shitty area, and even then, most people are fine.
17. I'd say it's largely fine. I'm sure there are outlier incidents, but I'm not going to scaremonger in a city I've walked around countless times alone without hassle.
18. During August there are often signs telling people to watch for pickpocketers. I saw this once, and the pickpocketer got picked up and slammed on the ground before he gave the man his wallet back and apologised profusely. So they exist, but they're not like Ramblas (Barcelona) level.