The K20 is trash. Buy the S8 now is my advice. The screen is huge so I see no reason to go for the +. You could always wait for the S9 obviously but I wouldn't want to use such an inferior phone for that long.
I've got 2: The Samsung battery backpack and the Olixar X-Duo. It might be the shape of the phone but I'm yet to find a case I really like the look of. Still, I am happy with both.
Bought my SecondSkin Edge from here. It fits with my Olixar case and has't made the screen less 'crisp' unlike some I've had in the past so I'm content. Would buy again.
I've had it for a while now and I'm still can't believe how much of a step forward it is. The fact that the useable screen size is so big yet it's nowhere near phablet size has changed everything for me.
There's nothing that I'm aware of but Greenify has helped me monitor my usage more closely. It'd seem like an easy app to create, maybe you've given someone an idea.
Sometimes with these apps you have to download a main app before you can activate individual themes or skins. That was the case when I downloaded a white keyboard and an icon pack anyway.
It was slightly noticeable on mine before the update, now even less. I'm not sure whether people are hyper-sensitive or whether some really are much worse than others.
The placement isn't perfect, granted, but it's already a non-issue for me. I can count the times I've touched the camera on one hand and the gestures and accuracy are so good I wouldn't consider complaining anyway.
I wish more manufacturer's went the modular route, but yeah the G5 wasn't the most popular phone of all time. Look at OnePlus if funds are tight, you can probably pick up a used 3 for around $300.