Well, if it doesn't boot at all (even if you removed the battery and reinstalled it) probably you bricked it. I had this issue once upon a time ( I also flashed wrong rom) and I took it to the store were I bought it (wind, hellas) and gave it back for repair. I had warranty so I didn't pay anything (I was waiting for my device about 20 days). Give it a try. Just say to them that it doesn't boot (and NOTHING ELSE). :)
Since I use dalvik cache, is it necessary to change in ART? Sorry for my questions, but untill now (except the message that popped up -I "fixed" it with reboot) I have NO problems with apps, or Google Store. Every single app works just fine. Sorry again if I'm getting bored for you. ^_^
I 've noticed this: wallpaper (from gallery, a custom foto, not a stock one),turns bigger (and stays that way untill I re-set it to normal size) when -from start screen- I press "settings" button (not icon from menu) and then again settings (icon that appears down right on screen). Some kind of bug I guess?