1. I’m not sure why the file sizes would be so different. I do not know how Camera+ does its decode or its export, so it’s hard for me to know for sure. However, it is true that RAW Power TIFF images on iOS are floating point, so they may not compress as well as integer ones. I am working with Apple on a way to create 16-bit integer tiffs using their libraries.
RAW Power generates 16-bit integer PNGs so that is an alternative — they are also losslessly compressed.
2. Pixelmator is able to open 16-bit floating point TIFFs from RAW Power — I just tried it.
3. RAW Power does not have all the features that you listed (like healing or brushing), so it cannot fully replace applications for customers that need those features. It is possible to add many of those features to RAW Power and most of them are on my list for the future. However, there are a lot of features on the list, so I cannot make any promised about when those might appear.