I'm a .net developer but I haven't worked on mobile devices. Could you make a couple different start screens that you could display one or another depending on the screen size? That should help your issue with making it look the same across different devices. (I run a Vogue with sprint for your records of what people are using... it looks great on the device btw)
I just downloaded and I look forward to trying it out today. When splitting is selected you could ask for the number of people in party and then after that selection, pop up the calculator where you have the person add in the prices for his/hers items and when you're done you click "go to next person" and add that persons stuff. Make sure to have first/last previous/next buttons on the bottom (you know |< < > >| style) so you can go from one person to the next.
I'm not a huge fan of the current menu system, but if you work in a start screen like what was posted, you could have stuff launched from that page and then have back to menu buttons.
For bugs: I experienced an issue when I set input to numpad when I clicked on the numpad button to make the keyboard go away, it would open my regular keyboard instead. I'm running Vin's rom on my vogue (
http://forum.ppcgeeks.com/showthread.php?t=50712) but I'm not on the very latest version of it so it may be a bug of the on screen keyboard. I'll let you know once I update it.
Sorry for being wordy, hope this helps!