Is it safe to use the Ferrata device to do a hold / rest ? Also it it ok to use the resting loop to place a hold sling, or Is it better to have a separate personal anchor system
Safety is relative to context. Yes in certain situations is perfectly safe, in other, not. There are many factors impacting safety while attempting to rest into the Via Ferrata Set. As a general rule, it is safe. It also is safe to use the shorter loop (where you have the lanyards of the Via Ferrata Set fixed to clip something that will serve for resting. Same discussion could be done here as previously. Is it better to have a separate resting system? Not necessarily. It again depends on multiple factors.
There is a whole theory of it. Via Ferrata Sets are made to be fixed to the belay loop. I guess it is as debatable as rock climbers never wearing a helmet.