Why didn't he lived till the end of this Kalpa himself ?
1st, the life-expectancy for human at this kalpa of this period is 100, Buddha lived till 80, which is within and fits in, to set example that a human can reach enlightenment, same as Buddha who is a human. 2nd, Buddha gave a metaphor in the Lotus Sutra, that a loving father (Buddha) in making his sons (students) to take the curing medicine (Dharma), feigning his death.
...if Lord Buddha was able to read minds and see future, why did he asked that question in the first place ?
To honour the Law of Karma, Buddha would not interfere with the course of events, for any intervention with a subjective purpose is going to flush away by the force of Karma, creates reactions, like an equation with only one correct answer. Thus to "see future", "read minds" is not applicable in this.
Buddha had hinted this 3 times to Ananda, however, Ananda missed all the chances. Then Mara Papman appeared and requested His nirvana, He accepted. Thus chances were given to Ananda, who the sum of all we students on the path of cultivation, a representative of all of us. The path to enlightenment is the overcoming of our nivāraṇa and Mara Papman by ourselves, Buddha could show you the way, but he couldn't do it for you. Thus Ananda must overcome Mara Papman to request Buddha's staying on. To staying on or to enter nirvana is the same to Buddha, who's no longer bound by life/death, black/white, cold/hot... but He will response to request. Same as, if you read the sutras, it always begins with a question asked by someone (Ananda was always the one initiated the teaching), then Buddha answered...