You may want to try my dkms psmouse module at https://github.com/kylerankin/psmouse-byd-dkms. I’ve basically just taken the patches that the Purism team has done against the Linux kernel and converted it into a DKMS-compatible package. I’ve only tested it against the stretch kernel but if you are using the backported jessie kernel you should be able to compile the module and load it.