How can I loop on the first n slides of a PowerPoint pres., but navigate to a last, UNlooped slide when I want to stop the slide show?

Ah, that explains it. PPSXs don't work correctly on PowerPoint for the web, they always open in edit mode. MS has been asked to fix this many times, but no luck so far.

The security notice is MS protecting their a**. I even get them now when I right-click on a zip file to open it. I'm not aware of any permanent way to defeat them. When someone posts a workaround online, they plug it sooner or later.

All files involved in this are on OneDrive. Running PPT from my laptop.

I have once again tried to use .ppsx's. PPT refuses to open those in show mode when they are linked to from a running pres (but do open in show mode when an Explorer listing the .ppsx is listed and I click on it).

However, all is not lost: I decided to try something a little different: Instead of generating a link (via 'Share') to the target file, I created an action object on the calling slide and inserted an action which is a "Link to another PowerPoint presentation [the .ppsx file]". And now, when I click on the action shape in the calling slide, the next thing I see is this:

I click 'yes' and the next thing I see is slide one of the target pres. in show mode and the loop is running.

(The warning popup isn't pretty, but it's of a minor inconvenience, and less disconcerting than navigating past a busy PPT editor screen. Any idea if there's a way to declare the hyperlink in question as safe, to preclude this popup from showing?)

Furthermore, I can now use ESC while the loop is running, and I am returned immediately to the calling [blank] slide. It's perfect!

Overall, there's something mysterious and dark going on with PPT's ability to link, how the links were generated and the file extensions, but I'm ready to meet my Saturday deadline am now breathing easier.

Another approach:

The first and last slides are one and the same, a presentation with only one or perhaps two slides.

On the first slide, add a link to a different presentation; this is the one that's set to loop forever.

You'll start the first presentation in slideshow view, click the link to start the looping show.

When you're done with the looping show, press ESC to return to the first slide/last slide in the original presentation.

If you don't want any visible links on the first slide, draw a shape anyplace you like and give it no outline, 99% transparent fill and assign IT the hyperlink.

In slideshow view you should be able to tab until you see a dotted line around the linked shape; press ENTER to start the linked slideshow.

You can create a transparent shape on the slide master that covers the entire background, then apply a link to it. Then you can click anywhere on the background to move to the other deck.

You'll have to click on it, there's no keyboard shortcut.

Create 2 separate presentations, one that loops and another that has your final slide.

Add a hyperlink to the final slide presentation on each slide of the looping deck. This is simplest to do if you add a text box containing the hyperlink to the slide master. Then, when you want to go to the final slide, click on the link.