1) You should consider updating to 10.10.5 Apple fixed some big bugs in Yosemite that were affecting After Effects in the 10.10.5 update
2) You might consider updating AE. The updates to 13.7.2 fixed quite a few bugs.
3) Importing image sequences is one of the nice performance improvements in the newest version of After Effects. Make sure, when you're installing the "update" to CC 2015.3 (which is actually a brand new version, so think of it as CC 2016), that you don't let the Creative Cloud app default to removing your older versions. CC 2015.3 installs alongside CC 2015, CC 2014, CC, CS6, or any other versions just fine with no issues. And project files can move back and forth between AE versions CC 2014, CC 2015, and CC 2015.3, so even if you run into an issue with the new version, you can simply open up the older version and get back to work.
1 and 3 are the biggies.
My first suggestion is that you keep working versions of Adobe products on your system so you can revert back with a single click if you have problems.
Are you telling me that you can not simply click on one image in a sequence and select Import as a sequence? It's working for me but I"m on OS X 10.11.5 and
Also, the latest build of AE (15.3) has an entirely new way of importing image sequences that is a lot faster.