There are these little brass sleeves that adapt splined shaft pots to accept set screw knobs without wonkiness or damage to the shaft. You can get them at All Parts for sure, And maybe at some other retailers. In my experience they work very well. They are also pretty affordable if I remember correctly. They do charge a lot for shipping there, so if your are going to order them you might save some money by ordering them in a batch with a bunch of other stuff.
As for getting push-on knobs off of push/pull pots, the first key is to not install the knobs in the first place if they are a tight fit. If you have no choice but to use tight knobs, you can still prevent ruining the pot upon their removal. You can do this by grabbing the shaft tightly with a hobby-sized needle-nosed pliers, or with the end of a pair of wire strippers. Alternatively, you can just slowly work the knob off by rocking it back and forth, working it up bit by bit.