1. Between 2011-2019 Google was giving installs and traffic to indie apps / games in massive scale. 100k installs in 1 day? No problem!
Now, you need to invest in ads to get userbase. Forget about 100k-500k installs from store itself. If you get it, it's just temporary luck
Most of traffic is going to big brands or social media apps like TikTok. Hype for apps is over. People just don't need apps.
2. Yes, if you can buy IDs from somewhere
3. Yes, if you can find it cheap
4. Probably yes, reskin time ended in 2015. From that time, you need to make not reskin but recoding, changing huge amount of code, so it's better to just make own apps