Hopefully you've already found your answer but for anyone else that might pop in, you can manually edit the player tab order. While on a slide view, click the Home tab from the main menu. In the second block of tools (the group is titled, "Slide"), there is a Tab Order button. Clicking that will open a window that lists all the objects within the slide that are viewable with a screen reader. On the left are the objects from the slide. On the right is the alt text (if applied) associated to an object. By default, the order is probably using the order of your timeline and Layers panel, so it might appear chaotic. If so, you can click the upper Check Box "Create Custom Tab Order." Now you can click on an object to highlight the row, then use the up/down arrows near the bottom of this window to change that object's position in the read order (top to bottom).
You'll probably notice a lot of objects on your slide that probably shouldn't even be seen or read. Stuff like random shapes, non-informative images, decorative elements like strokes or whatever. In those cases, close out of the Tab Order window and find those objects within your slide. Right-click on an decorative object (or whatever), and choose "Accessibility..." from the context menu. Within the new pop-up window you can simply uncheck the box for "Object is visible to accessibility tools" With the check checked, that object's alt text will be read. With the check UNchecked, that object will be completely ignored. GROUPS and button sets are another can get complicated, but the concept is applies. Doing this will remove your decorative object from the Tab Order list when you open it again, making your job tons easier.
I suggest saving often! Storylin360 crashes. a LOT. Half of those times seem to be when opening or closing the Tab Order window or the Player edit controls. YMMV.
But yeah, Storyline360's actual support for 508 is wonky at best.