For the touch issue I've been using the device for 4 months now and I hadn't faced it, it may has caused some taps when scrolling , but it didn't bother me and it doesn't happen frequently , really it rarely happens to me , the size is just a matter of time and you'll find it just amazing, everyday you'll get just more used to it, it's hardware is not old at all , the thing that might be better is the upcoming snapdragon 810 but do you remember what they told us before ? Even snapdragon 800 is more than enough for windows phone , so you definitely don't need the 810 I can say , and Microsoft is continuing great support for the device so I don't think you'll feel it old
I'm dying for a green cover too :') but its colors are good anyways
I guess you wouldn't regret getting a Lumia 1520 , remember Lumia 930 is the latest flagship and it has the very same specifications as Lumia 1520 so the support of Lumia 930 is gonna take a while and they don't have any reading not to support Lumia 1520 the same way since it is the same device indeed (except for the size and SD card and display technology) hope I helped and made you make the right choice