Here's the answer (kinda) to the Q about special abilities: dialog with Support.
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Me:
Can you tell, what is the exact effect of each of the special abilities acquired by the players? such as "dribbler", "defensive wall", "playmaker" etc.
Nordeus:
thanks for contacting Nordeus Support.
If you want to have more informations on Special Abilities, please take a look at this link:
Special Abilities in Top Eleven - Be a Football Manager
Me:
Thanks; can you give little more information about "By teaching them to your players, they will have a slight edge in certain situations" - how much of an edge?
If there are two players of the same quality, and then I spend 40 points to fully develop one player's special ability, and another 40 points on another player's general skills, which player will be more skilled?
Nordeus:
a special ability doesn't increase the market value of a player but your player will be more skilled only in certain situations!
So, if a player has special abilities is not necessarily stronger than a player without them!
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Reading between the lines, the answer is clearly that it's not worth it. The Unofficial Guide by Julie 4th concurs. I myself also saw "free kick specialist" missing one free kick after another, and "corner specialist" making no difference in a corner. If any difference in the probabilities game engine takes into account in these situations exists, it's tiny.