The project python path is appended to the python path, so this would not solve the problem. The venv site-
packages path has to appear before (or instead of) the standard python site-packages path. I don't think is such a major issue to change or add one entry in the python path every time you wan to switch venv. The Python path tool makes this quite easy.
Regarding your second question, to close an open project you just press the New project button or the Open project button. If the existing project has been modified, you will be prompted to save it before proceeding with the chosen action.