Finale 2012, 25 - Mac OSX 10.11.6
I agree with Adrian. X noteheads in the 3rd space.
If scrolling playback suddenly stops, just quit and restart Finale. It's worth a try and might get you up and running quickly.
You can replace a notehead with a symbol from another font. Use Special Tools > Note Shape Tool. Click in the measure, right click on the handle of the notehead you want to replace and choose Edit....
Files made with Finale's Graphics Tool will not show any layout icons. Screenshots will.
The 2012c update should be available here: http://www.makemusic.com/downloads/
When you install Finale, it automatically installs a folder for backups and by default any saved files go there. Try doing a search for "Finale Backups" or for the title of the score. If you didn't...
The only way to find out for sure is to clone your present setup onto a external drive. (I use Carbon Copy Cloner.) Go ahead and install Sierra. If anything is amiss, use the clone to revert to you...
I have seen a similar thing happen when using "fixed" point sizes for text in a text box. If you're using fixed sizes you might try changing a text box to non fixed to see if that makes a difference.
I would reset the Finale preferences. See here for info: https://makemusic.zendesk.com/hc/en-us/articles/115002541107-Resetting-Finale-s-preferences-using-a-tool-crashes-Finale-etc-
Go to the Utilities Menu and choose Update Layout (or just type Command/U). Do the pages advance now?