I think the point is that the lich isn't going to just sit there and let the PCs attack it, even for one round. The idea is that it would be prepared for them....so when they charge it, they actually run facefirst into a wall of force, and then several specters float up through the floor to attack the PCs and the lich teleports away.
That kind if thing. If a lich is simply standing in a room waiting for the PCs to arrive so they can all take turns attacking each other, then just use a skeletal champion or whatever because you are not doing the lich justice.
First, none of that affects the lich's CR. A CR 1 guy (albeit with with the right magic item to cast
wall of force) can do that. That doesn't mean the surprising intelligent and rich
commoner is an appropriate challenge for the party or mean its effective challenge is twice as high as it should be.
And it doesn't mean that when the party does finally get to him, he's going down in one or two turns.
Second, going through your situation, the lich casts the
wall of force then the next round teleports away.
This assumes that the lich beats the party with initiative and can block all of them with the wall. And it presumes the party can't
teleport,
dimension door, Shadow Step,
misty step or otherwise get passed the wall of force in the intervening round. Those are both pretty big "ifs".
Third, really, if played smart he shouldn't even be in the tomb. Unless the party makes it through undetected, he should just bail, activate the self-destruct, and set-up shop elsewhere. Or treat it like a pest control problem: seal the tomb and bug bomb the entire dungeon: a party that can't breathe isn't a problem, and as an undead he's unaffected,
But, like dragons dropping boulders on the party from 650 feet in the sky, that is no fun for the players. There's a balance that has to be struck between smart villains and fun villains.
And, since it's a published adventure, you have to reply on the presentation in the book, which is generic and not custom built for your party. What Chris Perkins decides might work, might be easily bypassed by your party. You can customise for sure, but that still relies on you (one person) being smarter and more cunning than 3-5 other players working together.