Hello,
We are glad that you liked it.
Small tests would have answer many of your questions.

> You can edit original path of the group by double-clicking on the group (as
> usual).
But what if I want to enter the group instead?

Actually, this is what it does : enter in the group.
 
> You can make groups of groups, it will work (There is one known bug when
> using bend path, we will fix it very soon).
>  You can apply definitively the effect with "Convert Object to path"
Does it convert a group with LPE to a group without LPE applying the
LPE to its paths, or does it convert it to a single path?
 
It convert a group with LPE to a group of path.

What if a group contans non-paths - texts, bitmaps?

You just can't apply the LPE
 
> We are currently working on LPE stacking (a simple UI is done, the engine is
> in a good way) and on an envelope deformation effect (it uses LPE
> PointParams for control points).
That will be very nice to have. Does it do curvilinear envelopes?

For the time being, an algorithm with 16 control point is being implemented. But I am experimenting an other one with curvilinear envelopes.

Thanks for your support
Steren