How to display icons in finder in newest to oldest order in a grid?
To sort by modified in icon view:

Go to the settings for your finder window by clicking on the gear icon at it's top and choosing View Options. You've already set it to arrange by date modified, but you also need to set it to sort by date modified. 

Then you'll have groupings to minimize until you reach the date range you are looking through AND a list of items to scroll through that's sorted by date.

*as an aside, in figuring this out, I noticed that the sections aren't entirely accurate for me in 10.9.5 (items from 2014 are in the 2013 section), but the files are in the correct order - so don't necessarily trust the section headers. Interesting glitch. 

As a tip - You could change to list view to see where the modification dates are about where you want to be, select an item to mark your place and then switch back to icon view. Another note - to find where you'd clicked after clicking show all, just arrow key to the left or right and your view will scroll to the selected file.

Hope that helps!