1) Toppers will keep a very long time
Don't put them in the fridge though as they'll go tacky and odd!
2) Two things could be at work here - you could be not getting all of the batter out of the scoop each time, thus different amounts, and also your oven temp...
I used to use a scoop and found I had similar problems - I now use a disposable piping bag and find it much easier to control the batter volume. It's trial and error temperature wise but I find my oven bakes best on 140c in the top non-fan assisted, and on 120c in the main fan assisted oven, takes a while longer but any peaks are rare and it maintains a lovely even colour.
3) Mine's quite thick and I pipe it out, even if there are peaks in the mixture I give it a tap on the work surface to even it out a bit - the low temperature tends to help with this though as the mixture warms gently and settles out!
Thought I'd pop on and share some pictures of the birthday cake I made for my OH's birthday today - sorry if they are a little large
https://a7.sphotos.ak.fbcdn.net/hphotos-ak-ash4/300017_10150333457098562_503143561_8062528_2124322783_n.jpg
https://a3.sphotos.ak.fbcdn.net/hphotos-ak-ash4/301690_10150333553003562_503143561_8063436_1257328707_n.jpg