Yes, the V2.7 release is based on the morty branches.
The angstrom-lxde-image recipe does not build or pull in anything from meta-qt5. The meta-qt5 layer is added because meta-angstrom depends on it and bitbake’s recipe parsing would fail without it. Note that the layer has been present also in the V2.6 releases based on jethro.
So you will need to build your Qt5 based image yourself, that build will use Qt 5.7 from the meta-qt5 morty branch.
For the T20 and T30 based modules the hardware accelerated gstreamer integration is based on gstreamer version 0.10. How that fits with Qt 5.7 I do not know.