And does gas drag tend to make a planet spiral inwards by reducing net centrifugal force?
Mmm, I wouldn't describe it that way; it confuses the physics involved.
Any phenomenon whose explanation invokes centrifugal force may instead be more directly explained by centripetal force. The centripetal force is the real force, whereas centrifugal is the perceived force due to observer being in a non-inertial reference frame. So there is no need to use centrifugal force to explain the effect of gas drag; or even to explain planetary orbits at all. A planet is held in orbit because gravity, a centripetal force, pulls its otherwise straight-line trajectory into an ellipse. A planet experiencing gas drag spirals in because it loses orbital energy via friction with the gas. You can think of it as an object moving through fluid.
I can't see your image by the way. It appears to be a wolfram alpha link, but the page doesn't exist. But yeah, your description sounds right. It'd be analogous to the orbit of a decaying satellite, except satellites encounter exponentially more drag as their orbits decay and so their spirals widen significantly.
Also, what is the typical outward velocity of a planetary nebula? I.e. does it move slow enough to allow for significant decay in a planet's orbit?
Outward velocity of planetary nebula is complex and non-constant. That being said the migration due to gas drag from planetary nebula is probably insignificant compared to migration due to mass loss during the same phase.
I also have to say that you might be putting way more thought than necessary into figuring this all out. The migration of planetary orbits through stellar evolution is really complex stuff! People investigate it with detailed models and simulations and even then the results have large uncertainties. I would recommend just sticking with the results we found above; they're not bad as a first approximation.