Wait for me

Fortunately, the German guards didn't notice when Corporal Young struggled to empty her tunnel dirt onto the courtyard.

Having brazenly concluded that the reader should be primed immediately before a major reveal, I must now justify my claim.
Here's what I reckon.

It's likely that any fresh revelation is gonna require of the reader a bit of thought: suddenly, the reader must apply this new knowledge to all that has gone before.
Ah, so Vader is Luke's dad! Let me think about how that works ...
In this example, Luke and Vader bash out this revelation together. Once the loose ends are tied up, Luke makes his decision and falls to almost certain doom.
The movie waits for the viewer to catch up.
However, a more mature audience probably won't want, or need, an explanation of the effects of a revelation.

The reveal is a critical moment. You get one stab at it. If the reader is primed and the reveal is freshly set-up in his head, then it will instantly hit home. Emotional response. Yay.

For an example of what not to do, let's take a look at Saw IV.
What an awful ending. Hit with one reveal, my mind started dealing with the implications, but was given no time to do so before it had to deal with a load more nonsensical and supposedly climactic moments (I knew they were supposed to be big deals because that music was kicking off). The film gave my brain too much to process at once and I found myself several steps behind the revelations.
You get one stab at them. So be quite sure that the reader is precisely where you want him, and then allow him time to adjust.
By the time the Saw IV credits began to roll, I had no idea what had just happened and, with no more movie to watch, cast the thoughts from my head and got on with something else.

All of these thoughts are leading to my big question:
How should I conclude my first act?

I figure that, with a hypothetical down-time between acts I and II, I am entitled (and perhaps even expected) to present the reader with some sort of revelation that will require some thought. There aren't many places where I can easily do this in a novel. For one, a major revelation would then require the pace to slow (so that the reader can catch up). For two, a major revelation would be hard to eclipse later on. Little reveals, for sure, but stuff that creates so much change, be it a major reveal or a reversal or whatever, needs some consideration from the reader.

I'm cool with this theory, but ...
I'm giving serious thought to shifting the negative opening to act II - also an event which requires of the reader a reasonable amount of contemplation - to the end of act I, such that it follows the positive ending I have already.
In principle, this would give me a lovely, rapid-fire positive to negative switch, and two key moments back-to-back. And, I've figured out how to use an incongruity between these two events to create a humorous and ironic conclusion. Importantly, the reader is invited to reconsider the course of the hero's journey and to reasses how it might continue. And anticipation is always the key (anticipation = page turning).
Provided that the first trigger doesn't require too much immediate contemplation, my plan might just work.

In considering how best to conclude an act, I guess it's worth considering the nature of this virtual void, this limbo, which reaches from the end of one act to the beginning of the next. In this limbo, we can dare to gift the reader with things that we wouldn't dare elsewhere.

I have thoughts to come on degrees of space (limbos). In his essay on the elements of style, Robert Louis Stevenson refers to a web of phrases bound by rhythm and meaning. Consider the breathing spaces that reside within commas, semi-colons, full-stops, line breaks, and all the way through to the inexorable void that sits at the novel's end ...