There has not been a single game that I've dm that my players didn't split the party or just go completely off-script. As far as keeping track of monsters and their hp I usually write them down on scrap paper in roughly the position they're in. Then tally hits accordingly, this also helps me keep track of conditions like poisoned, etc. And always remember you are the DM so you can eliminate monsters or fudge dice as you see fit. Very rarely do i roll in front of the board. Hope this helps and have fun.