1 - Is it possible for a 6'66 "/ 1.98 cm player to become a PF or would he be more suitable for SFs? I ask for the speed of the training.
Players who are 6'6" can definitely make a good PF but are ideal for SF. He'll take longer than a taller player to train in inside skills, but quicker for outside skills.
2- What is the real use of Shot blocking for a player? I see players with high levels, but to what extent is this really necessary? Would a 12-12-12- or a 10-10-10-10 player be better?
There's been a massive discussion on Shot Blocking over the years. Some managers dismiss it, some managers swear by it. Try this forum: https://www.buzzerbeater.com/community/forum/read.aspx?thread=161952&m=11
3 - How high must the rebound of a high level SF and PF be? And taking advantage of the question, is a SF with a high rebound a bad thing?
Obviously rebounding will need to be higher for a PF. If you're aiming for NT level, I'd expect 15 or higher. For a SF, I'd be happy with 9/10.