Learn English – -ing form after “can’t help but”

In your test the correct answer is, probably, passing. That's what Murphy's English Grammar says:

When one action happens before another action, we use having (done) for the first action:

Having finished her work, she went home.

You can also say after -ing:

After finishing her work, she went home.

So, when there is the word after, it is more "right" to use [verb]-ing. But note that after having (done) occurs in practical usage. These sentences I've found on Google News:

After having done this, he returned to the scene

A bus driver almost choked to death after having found a coil in his two chicken breasts

Khan thought he had the fight with Mayweather wrapped up after having won a poll on the website.

Ahmad, it follows the same rules as the past question. All tenses mentioned can be used in that sentence. However, as it's a past tense and you are telling a past history, using "had" would make it sound more fluent.

I waited until he had left the room, then I entered there.

Now talking about your second question.

I am waiting until he comes back - Right

I am waiting until he will come back - Wrong

I wait until he comes back - Right

Remember, "until" cannot be used followed by the future tense, all the others alternatives are suitable for until, past tense, past continuous, present tense, present continuous, present perfect..

And I waited for him to leave the room also works and sounds good too, and functions with others tenses too:

I was waiting for him to leave the room

I will be waiting for him to leave the room;

I waited for him to leave the room

I have been waiting for him to leave the room.