Then it seems to be an issue with the tiles that PowerVR uses for rendering. I would say it's a bug of some kind...I'm not sure what to do about it.

Might be an issue with rendering tiles on PowerVR. It would be interesting to know how wide these blocks are. I expect them to be 32 pixels...

I don't know, because I don't know what your current solution can do "out of the box" or what your actual requirements are. jPCT provides means to do the projection 3d->2d as well as blitting, so that part is rather simple (a few lines of code should suffice). Another question is how the user is supposed to attach the annotation? By tapping on the object? By some other means? If it's the former, jPCT provides means to find a point on an object that corresponds to a screen coordinate as well, so that's rather simple to solve as well.
What jPCT doesn't provide are loaders for STL or DAE but you should be able to port yours over to it. Again, it depends on your actual requirements, so I can't tell if one way is "easier" than the other...

Well, this is a jPCT related forum, your solution doesn't use it. However, the basic idea is to take point in 3D space on which you want to "attach" the annotation to, project that into 2D screen space and then somehow blit the annotation as text on that position. Either that way, or you could "attach" another object at that point in 3D, make it inherit the actual object's transformations but render it as a bill boarded quad with the annotation's text rendered onto it. That has two drawbacks: It's much more complicated and it scales with the object which might render the text unreadable. Anyway, I've no idea on how to implement any of this in the context of your current solution.

Yes. That's caused by the way in which these objects are build. I think ExtendedPrimitives does it differently.

IIRC, you have to create a GL context with an alpha channel and clear the framebuffer with the RGBColor variant of the clear method to set an alpha while clearing. There should be a simple example of how to combine the camera view with a rendering on top, which is a similar task, somewhere here in the projects section. Though it's hard to find for me ATM too, because I'm on mobile right now...

Edit: There's a sticky topic here in the support section about that camera overlay thing...maybe that helps.

If it's a problem to run it from Google Play, I can always upload a side loadable version to my website if that helps. The game continues to grow on a slow but steady pace. Next year, it should surpass 500000 installs. Sadly, Google Play doesn't show the number of total install anymore to the creator for...reasons, I guess...