1. The problem with apply global tweaks such as these is that it usually results in messing up the overall ratings - and the generated rookies will be much better than the tweaked players. However, it's easier to untweak players globally than one by one.
2. Editing the jumping rating will not do much to fix up the blocking problem. In fact, you'll basically create a new problem - too many missed dunks.
3. In my experience, ratings cannot be set over 99 using the Mega Modifier.
I'll warn you now, tweaked rosters really aren't suited for Franchise Mode, and although a lot of people do like to use DK's Roster, Zaxis' roster or the tweaked version of my NLSC roster patch, they really aren't suited for Franchise because of the long term (and short term) problems they cause. But if you feel tweaking is the only way you can enjoy playing the game, best of luck with your tweaking.