Since you are using Jessie images, have you tried using ConnMan to manage the WiFi connection rather than resorting to all of this systemd and ifup/ifdown stuff?
You can configure ConnMan using the "connmanctl" command line utility, or via the "cmst" GUI utility. to setup your link (ssid, credentials). Or, you can create a config file and put it in /var/lib/connman/my_config.config. There is documentation in the ConnMan source tree on this format. I've also attached a template for reference. You might also consider adding '/etc/connman/main.conf' and specify PreferredTechnologies in the order that you want. Again, there is documentation in the ConnMan source tree on this file, and also the Google will help.
Note that if you use this method, you should NOT do any wifi configuration in /etc/network/interfaces as ConnMan will override these. If you would rather use /etc/network/interfaces, then be careful that you don't also have conflicting configuration setup with ConnMan, as ConnMan will override configuration in /etc/network/interfaces.
I use the ConnMan method all the time for my work and have no problem getting a WiFi connection after boot using my TP-Link TL-WN-722N (uses an Atheros chipset), Jessie snapshot images, and 3.19.x kernels.
ba