Your app
At the end I didn't understand whether you found what you are looking for or not. My opinion is that the only chance against hackers is that they will not be interested in you. Normal precautions should be taken, of course, but I don't think this the biggest problem, in your case. It is my humble opinion, of course. I have just developed an application (with b4a of course), that is someway similar to yours. (Screen shots available, because I plan to sell it on Android market). A database on the site and a b4a application sending and receiving data to it. I am using either Access or SqlServer, though only the first has been tested. I did two "interfaces" on the server, i.e. a classic asp and an asp.net/c# application, just like Erel's tutorial, but independently. B4a app manages data through one of these apps (classic asp showed to be less problematic on remote publication. I still have publication problems with aspx). My opinion is that there could problems on some "details" in your app. If you don't find any "guru" and still have this need, I can cooperate with you. I have time and I am interested in developing on mobile devices. I come from C++ programming and I found b4a a very good product, really. It was easy to make this app. Though there could be some "impossible" things, for now. For example, I discarded the idea of making a video analyzer, because till now there is no support to this (at least I have seen this, some time ago). That's all.
Best regards
Giovanni