We will arrive at 22.55 pm at Beijing Capitol Airport T2. Can we reach Qianmen by bus or metro?

You can take the airport shuttle bus line 2 to reach its terminal at Xidan and then take a taxi to reach your hotel.
You can catch the last train of the airport express, but upon reaching Dongzhimen Station, you cannot catch the last train of the local subway line 2 to reach Qianmen. So I think taking the airport shttle bus is much more easier for you.

But is it possible to catch that train or bus within 20 minutes? I don't know how long it takes before luggage arrives.

Please do not worry. The bus line 2 will operate till 24:00 and after 24:00 there is still a night line shuttle bus taking you to reach the city center.
But it is hard for you to catch the airport express line within 20 minutes.
In my opinion, it is suggested to take the shuttle bus instead of the train.

Thanks!