There's going to be a range of velocity...a guy like Taillon will probably go top 5 overall and I've read reports of him touching 99. There will be other pitchers who are more likely so sit in the low 90's and occasionally throw up a 94 or 95. As you get into later rounds, you will even see guys with lower velocities (say sitting 87-88 touching 91) who teams will take a flyer on because of their projections. Most of those guys will probably end up in college to take the chance that they add a few ticks of velocity over the next three years and can really improve their draft status.
Being a lefty is an advantage, because left handed pitching is a more scarce resource, but I'm not sure I would say its a huge advantage, at least at the top of the draft. That will generally mean that a LHP can afford to throw a little slower than a RHP to get a comparable grade.
To answer your final question, no a team can't hold a player's rights through college. There is a deadline around August 15 for all players that aren't college seniors (I believe) to sign or teams lose their rights (and, in fact, the draftee must consent to allow that team to redraft them in the future should they fail to sign). This is somewhat new...until a few years ago the rule was that a team lost the rights to sign a high school player either the day he attended a college class (if at a four year university) or one week before the following year's draft (if a junior college player).