The correct answers are Agent Bob was going to steal the microchip, but he accidentally locked himself in the bathroom, He was to have disabled the laser grid at midnight, but he fell asleep, and He was about to escape through the air duct when his trousers snagged on a screw.

"Was going to + base verb" is perfectly used for unfulfilled intentions in the past.

"Was/were to have + past participle" is a formal way to talk about an arrangement that did not happen.

"Was/were about to + base verb" describes an action that was on the verge of happening.

Why are the others incorrect?

We cannot use "would" for unfulfilled personal plans or intentions (we only use it for predictions, promises, or narrative destiny).

"He was to disabling" is grammatically incorrect; it must be followed by the base verb ("disable") or perfect infinitive ("have disabled").