No, what you propose is not legal.
The LGPL license is written in such a way that the linking exception only works one way.
All the code that an LGPL library depends on must be under a GPL-compatible open-source license
Code that depends on an LGPL library may use a GPL-incompatible license.
The fact that the LGPL library gets used by a GPL application is ...
Yes, that would be allowed.
Versions of a software product that don't derive from/use GPL code are not subject to the GPL licensing terms, so you can use whatever license ou like for those versions.
If the people using the development builds are all part of the same organisation, which holds the copyright on the project, then there the use of the GPL ...
Mandatory disclaimer: I am not a lawyer.
As far as I understand it, GPL allows your program to depend on non-GPL "system libraries", which are:
The “System Libraries” of an executable work include anything, other than the work as a whole, that (a) is included in the normal form of packaging a Major Component, but which is not part of that Major Component,...