1) There is, but only for any Gen 1 Tamas you raise (good care you get Mametchi/Lovelitchi, poor care you get Shimagurutchi/Momotchi, etc.); ater that, as far as I can tell, the characters genetics are random rather than being influenced by care misses. This makes it very easy to play, as you're not constantly trying to get X care misses for a particular character.
2) There are eight in total. In order, they are; Tamatown, Patchi Forest, Flower Hills, Mermaid Palace, Rainbow Hills, Gozaru Village, Berry Town and Spacey/Melody Land (depending on which version you get). A guide on how to unlock them can be found
here.
3) There are eight built in wallpapers you can buy (plus the defaul wallpaper, so nine in total). From what I can see, each wallpaper is themed around each town (so there is a sweets one, a forest one, a rainbow one, etc.). You don't need to have unlocked each destination to use the wallpaper, they're all available from the beginning.
4) It took me a little while to get used to, but this is also because both my iD L and my P's are in English, so not only was everything in the wrong place I couldn't understand what it was saying! I started it up on a Friday and I had familiarised myself with the new layout by the end of the weekend; I sometimes still accidentally click the wrong thing (such as selecting the restaurant instead of the fridge), but nothing too major. Things aren't necessarily in the places you'd expect them to be (e.g., the arcade is in the TamaDepa). It's a bit more like the 4U/+ than the P's, but it's still very simple. The English manual (which I linked you to in my previous post) is very useful for working your way around the menu. The Tamagotchi also 'feels' different to the P's, as the animations are far more details and the sprites are bigger; there's nothing wrong with this though. It's really lovely to look at, definitely the best Tamagotchi in terms of graphics. (If anything, it'll be a sruggle to return to older models with smaller, less defined sprites).
5) Yes, it is. You have to unlock the destinations by fulfilling certain criteria, but there's no Sticker/Puzzle/Clover pieces to collect or places to unlock after X Generations/Tamatomo. I feel that the M!X is less of a 'game' with unlockables and achievements than the P's, and more like its ancestors, the vintage/Connexion Tamagotchis, in that it is 'just' a virtual pet (if that makes sense).
6) The M!X has got rid of clothing, but you can still buy accessories to wear, such as hats and glasses. Tamagotchis cannot wear the engagement ring(s) that you purchase; these are just for marrying Tamagotchis off.
Hope this helps! Please don't hesitate to ask me more questions if you have any! ^_^ The M!X is good fun and I'd definitely recommend it.