"4. Place a checkmark next to the "Unknown sources" check box. "
I understand this is required since the flash APK is not being installed via the Google Playstore. However the instructions end at step 16:
"16 When installation is finished, tap Done."
New steps 17-19 is urgently needed, once Flash is installed successfully the checkmark next to "Unknown sources" should be removed, restoring the security of the device to the orginal settings before following these instructions. Proposed step 17-19
17. Open the Settings menu.
18 Select Security (or Applications, on older Android OSs).
19 Uncheck the option next to the "Unknown sources, Allow".
I'll submit this suggestion to Adobe PSIRT if no action is taken in the next couple of weeks.