前 or 後 when placing an event in temporal reference to “now”

Question

Consider this phrase:

　今は(event)二十分前です。

Deconstruction of the above phrase yields:

Now is twenty minutes before (event).

(Q1) Can I reconstruct the statement using 後 in this manner?:

　今は後で(event)に二十分がある。(I'm not sure if I used に correctly here)

(There are twenty minutes from now to (event)).

(Q2) Can 今は and (event) be elided once the context is set? That is to say 二十分前です and 後二十分がある are acceptable phrases?

I think that (event) cannot be elided to form 二十分前です and yet retain the meaning. By itself the phrase would mean "(it was) twenty minutes ago" and would be too vague to be useful to specify (event) in relation to "now".

Answer 1

Notice that はis a topic marker, and there is often a corresponding form without the use of such particle. In this case, the correct standard form will be using から

今 から その式まで時間が後二十分ある。

1. When you topicalize a modifier (as opposed to an argument), the postposition cannot be omitted, so it has to be 今からは, not 今は (except when you use 今 adverbially).
2. The subject 時間が is omittable. But note that 二十分 is adverbial, and does not take が.

So the form closer to your example is

今からはその式まで後二十分ある。

You can elide 今からは

二十分前です。
その式まで後二十分ある。