Hi, and welcome! As far as I know - except for a few premium lenses with their own AF/MF switch - the only possibility to switch to full manual focusing is the small lever on the left of the K50 body. I don't know how flimsy it is - as I don't have a K50. (I suppose it is likely very similar to previous models - which were quite solid.)
About the quick-shift: Quick-shift is a special feature on different lenses from Pentax - but not on all of them. Quick-shift allows a full time override of focusing before or after the AF was activated - without switching the AF/MF lever on the body (or lens).
If your 50mm "something" lens is a FA 50 f1.4 then you don't have this feature on it, because that was a lens manufactured before quick-shift was introduced.
The modern lenses have this feature, as DA, DA* and DFA lenses (see the lens database on this site:
Pentax Lenses - Pentax Lens Reviews & Lens Database
(there are also modern lenses, for example DAL lenses <budget version of similarly named normal DA-s> - which lack this function)