1) Touching a sparking conductor: Here is a picture of a typical vehicle spark plug:
You can see that the plug has a special structure which facilitates sparking - a pointy head and a very small gap between other terminal. Also, whenever there is a spark, it doesn't mean there is a huge electrical current flow. When you take off your sweater in winter, you might hear a crackling sound (and see sparks if you are in a dark room) but you don't feel anything because there wasn't a lot of current. Besides this, current will always follow the most convenient path. If you touching the wires going to the spark plug, you won't get a shock because current will flow through that specialized gap and not through you.
2) Lighting of screw driver (tester) even when you are wearing plastic trainers: Remember that you are working with ac here. ac can also pass through capacitor besides resistor. When you are isolating yourself from ground by using plastic trainers or carpet, you are just increasing the resistance for the current path. However, there is still a capacitance present by which you can conduct some current (and get shocked if you touched the Live wire during your evil experiments). If you compare two cases - One where you are standing on floor with bare feet and Second with shoes or something insulating, you will find a difference in the brightness (depending on what model you are using).
3) Hair dryer myth: This won't be as bad as being shown in movies where a person dies when a hair dryer is thrown in his bath tub. Applying the same principle - Current path will be the most convenient one. Current from Live wire will prefer flowing to Neutral directly because it's the most convenient path and close by. If you are kept in between those two wires, then you will feel a shock but not very bad shock unless you come in a direct contact of the Live wire. Here is a video demonstration for this.
Regarding those ico covered pylons, nothing happens because snow/rain contains almost no salt. Pure water is insulator. So, snow covered pylons is almost as good as air. Well almost because it has a relative permittivity of around 80 which will increase the chances of sparking but the gap between High Voltage and Ground must have been designed to take this in account.
4) Train surfing: Again the same concept as above. 25 kV will discharge if some grounded conductor comes within the distance required for a discharge. Either 100 mm is too large for discharge or that beam might be isolated from ground (chances of which are highly unlikely). A person however, if he is on a moving train, he might tip or somehow get into that range and roast himself.