You don't need to use command finder or customise to set up the QAT.
Just press the symbol at the right hand end of the QAT (looks like a down arrow with a line above it), and in there is the pre-defined list of options for the QAT which includes new and open). All changes made this way have worked fine for me.
I've just tested using command finder for a command that isn't already in the pre-defined QAT list (I used extrude).
That stayed on the QAT after switching application to drafting and then back to modelling.
I suspect the issue is that you can't have more than one copy of the same item on at least the Quick Access Bar and therefore since the the 'File New' and 'File Open' items are technically already assigned to the QA Bar, just not activated, when you attempt to add one of these manually, at least via the MB3 non-customize route, they are eventually removed by the system.
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA Siemens PLM: UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.