It takes more power to decelerate than it does to merely hover.

Absolutely, especially with all that fuel mass.

There's an optimal curve of engine power / fuel consumption.

Yes, and when you're trying to minimize your fuel load you find and fly that optimal curve.

there was a "no-abort interval" during the LM descent. ... My question is, was this true, and if so, why?

Yes, it's true.

A staged abort is where the descent motor is turned off, the thrust is allowed to decay to zero (to prevent recontact at jettison), the descent stage is jettisoned, and the ascent motor is fired to effect an ascent.

As soon as the descent engine is cut off, any upward thrust ceases. The spacecraft immediately begins to acquire downward velocity at 5.37 fps per second. The staging of the LM is not an instantaneous event, and every second that the LM is in free fall is an additional 5.37 fps that must be arrested and reversed by the ascent motor. Even if the LM were hovering, with no downward velocity, there is an altitude at which it would be too low to effect the staging before it hit the ground.

It is not so much a technical limitation as it is a physics limitation. Pilots face these kinds of limitations all the time. The "dead zone" of the LM is not unlike the decision altitude when landing a conventional aircraft. To land, you must fly low and slow. There is an altitude below which the airplane will not be able to pick up speed and climb before it hits the ground. So when you drop below that altitude you are committed to the landing -- for good or ill.

Having a more powerful ascent engine, or some faster physical means of separating the stages, would reduce, but not eliminate the dead zone. Every craft has an operational "envelope" -- safe boundaries on the parameters by which it operates -- and you have to learn them and avoid passing outside of them.