The donationware Startup Mode Selector works well to always start in 64bit mode now and I assume it would work equally well to force a start in 32 bit mode. I am running in 64 bit mode full time and have yet to encounter anything that does not work. But I use Adobe Reader 9 and not the PDF browser plugin.
I could never make Reader work in other browsers than Safari; even in Safari, I had intermittent problems. PDF browser plugin works with all of them. I hope, the author will tweak it to work in 64-bit mode soon.
Alex 3.1 GHz 13" MacBook Pro 2015, 8 GB RAM, OS 10.11.2, Office 2011, TimeWarner Cable 2.8 GHz Xeon Mac Pro 2010, 16 GB RAM, OS 10.11.2, Office 2011, LAN
Unless you're affected by any of THESE fixes, you prob wouldn't notice anything. My guess is the bad press for the Flash security issue is the main reason they rushed such a small batch of fixes as a full .1 update.