It seems that these words are derivations from the noise one makes when startled. The spelling difference οι/υ vs. ο is explicable by whether the involuntary sound was perceived as produced with the lips in a slightly different position and with the vocal tract more open or closed. The upsilon spelling, which only has Koine attestationamong the examples in LSJ under πτύρομαι is also explicable by the emerging equivalence in pronunciation with the diphthong οι.
Reading these words as onomatopoetic gives a slightly different nuance to each of the four verses in the New Testament where they are used.
Reading this as the less intense, “startled”, “taken by surprise” gives greater coherence with the following information. His listeners are now expecting that these things will happen, so it should come as no surprise to them when they do.
Here reading this as closer to the startling that produces the verbal reaction, the natural progress of emotion is better expressed. “They got startled, then began to feel fear in their hearts”. Somebody first gets a startle then, fear begins to grow, as rationality finds the thing initially seen as contrary to nature.
Reading this as onomatopoetic rather than a deeper emotion of the heart, the impulsive nature of emotions is given greater emphasis. A softer meaning of almost “don’t be taken by surprise by anything (done) by those who oppose (the Gospel)” is possible.
Here, I think this is a variation on the same construction of meaning as in Luke above. Reading the πτόησις as onomatopoetic, ie. retaining a close relationship between the drawing or exhalation of breath and the the startling experience, the meaning is something like, “not be driven to fear by every little thing that startles you”.
Reading πτοέω as a vaguely defined “fear” is quite inadequate and open to a wide variation in interpretation. Reading it with the connotations of onomatopoetic interpretation gives greater clarity to the meaning.