I’ll try to explain some of what I see, and hopefully that will get you back on track. I’m looking at the full score layout.
On the first page, it looks like you have manually edited the page to insert your title and subtitle. This causes a page override (red triangle in the right-hand panel), which means that this page will no longer respond to changes to the underlying template. A better way to solve this is to open up File > Project Info and fill out the fields there as desired, so that the templates can pick them up automatically. You’ll have to right-click on p.1 in the Pages panel and choose Remove Page Overrides to get this to work.
You’re actually using the flow heading to display the subtitle, so you should edit the flow heading template by double-clicking it in the right-hand panel and changing the tokens there to {@projectSubtitle@}. (In many cases, you’d want instead to edit the First page template and add the subtitle token there, but if you have just the one flow, this is an okay way to handle things.)
I can see that on several pages you have have manually adjusted the space between systems. I would suggest doing Engrave > Staff Spacing > Reset Layout to get rid of those manual adjustments.
By default, Dorico will then justify (space out) your staves and systems to fill the page. I can see that on some pages, like p.10, this puts too much space between the clarinet and piano. One way around this is to go to Layout Options > Vertical Spacing > Vertical Justification and change the second number, currently 60, to 80. That will basically tell Dorico that when the page is more than 60% full, you do want it to justify, but you only want to add space between systems, not between staves within a system.
As for your missing piano part, if you turn on signposts (View > Signposts), you’ll see a signpost that says “SYS BRK + STAFF VIS”. And if you double-click that, you’ll see that you’ve set the piano staves to hide. Turn off those two toggles and click OK, and your piano staves are back.
Finally, the music from the beginning is now duplicated on p.18. In the Pages panel, there’s another red triangle on that page. Right-click and remove page overrides, and everything is back to normal.
If you don’t like how Dorico has spaced the two systems on the last page, then go back to Vertical Justification and set both values to 65. Now this page doesn’t trigger justification (because it’s only 62.5% full, according to the indicator in the lower left corner), so the two systems stay in their default positions.
SlipperySlopes-dor-REFORMAT edited.dorico (2.2 MB)
Edit: @Janus has done some other things as well to improve the look beyond just getting it back to baseline.