If all copyright holders agree, then yes (see mysql).

The main issue with that is that you have some problems with accepting external contributions - you need a copyright assignment process, which is difficult to do internationally and a hurdle for contributors to overcome.

It's a valid use of GPL in dual licensing context. Other examples of dual licensing are: VirtualBox, MongoDB, BerkeleyDB.

All patch contributors who initially chose GPL side of dual licensing model should sign copyright disclaimer for their patches, otherwise their code won't be accepted into the main repository. Alternatively, patchmaker may execute his/her GPL rights and fork the project (e.g. Percona).

Sure. If you wrote it, you can license it any way you like. The Qt framework is probably the most famous example of dual licensed (GPL and commercial) software. http://qt.nokia.com/products/licensing

A license is just that, a license to use the code. The copyright holder of a piece of code (or really any other copyrighted data) may license it to whomever they want, however they want. The GPL is just a blanket license giving permission to everyone provided they follow the restrictions put forth by the GPL.

If you are the sole copyright holder (because you are the only person who worked on a project, or because all others assigned their rights to you) then you are allowed to provide any additional licenses besides the GPL. A lot of open source projects (see Qt, MySQL and a bunch of others) work this way, only accepting patches from the community if the patches come with a copyright assignment. This allows them to charge for closed source use of the library while at the same time allowing the open source community to benefit.

It's actually GPLv2 or proprietary for "commercial use" at the choice of the user which is reasonably common.

Since "commercial use" can include cases where GPLv2 is acceptable to the user (either because you don't care about handing out the source code e.g. because you're selling expensive hardware or consulting, or because you're running it on a server so the GPL doesn't apply to you) you can't forbid those uses and offer GPL, but you can offer things beyond the GPL (i.e. keeping modifications closed, which is what they're trying to imply with "commercial use").