Re: Chord transitions
This may not work immediately, but in the long run it works great.
Try strumming pattern, any pattern, and keep doing it. Like Down-Up Down Down-Up Down for example. Don't worry about any chords yet, just keep strumming them all open for now. Play this rhythm until you know you're not gonna break it when changing chords.
Now, just apply a chord sequence to it. Concentrate very hard on not letting your strumming hand fall out of rhythm. Just keep strumming. Then, if your fretting hand falls behind by not switching fast enough, then let it catch up. Do not stop strumming though. I'm willing to bet that you are having problems because for that split second you pause strumming to alter the shape of your fretting hand.
Even if you fall behind with your fretting hand, it still sounds far more musical than if you fall behind with the fretting hand and pause with the strumming hand. So just keep strumming!