Hello there I am an experienced Unity3D developer. Though my role is usually in the development of the applications mechanics and functionality both in C# and JavaScript(UnityScript), I also handle 2D and 3D design, texturing, modeling and animations. If you would like to see some of the work I have done in any of the areas I have mentioned I can provide you with samples.
I hope I'm not being to to presumptuous, but I have read through your project description and the file you provided and find a bit of a problem with how you may be implementing how the user can design the cupcake.
First you did not explain whether if the cup design scene will have the cup cake viewed at a top down angle or at a side angle. If the cup cake will be at a top down angle then the process of painting with your fingers as you described should work fine for your average user.
Though if you wish to have the design done at a side angle, unless your user is a fairly skilled digital painter then there will be issues with how the painting can be done by finger specially by your average user and will result in an image no where near similar to the desired final images you provided. At a side angle it would be better to have preset model for the icing with adjustable parameters such as height, color, thickness, nozzle shape etc. this would give the average user a much easier time and more satisfactory result.
Sorry for the long bid description,
Please message me with any questions,
Thank You