If I'm moving back and forth between strumming and fingerpicking, I definitely use the open hand strum (where your thumb just sits on the bass strings and plays them when needed). The 'pretending to hold a pick' method to me works better when it's used for a stretch at a time when more volume and pick-like attack is needed. Sanding your thumb off sounds undesirable...
For a pinch (aka pick) harmonic, it's tricky! You hold your first finger and thumb like you're holding a pick, but curl your first finger just a bit more so it's against the last joint of your thumb. Now play the string (the G string 7th fret is about the easiest) with your thumb and first finger touching it at the same time (the thumb will be pointing towards your headstock. The first finger will be playing with the face of the nail, and the side of the thumb will be lightly touching the string. Now turn up the gain and squeal away!