How to set default action handler permanently?
AFAIK, defaults on Android is not preserved if apps that use the intent are changed.

Workaround:

Use intent to launch the application or its activity screen.

intent is comparatively reliable though it may be changed when app is updated.

Take sending email as an example


Install Intent Intercept (Google Play)
Clear the defaults of email client
Choose "Intent Intercept" in Share menu when sending email
Copy intent details of your desired application activity
Open Tasker to "Create Task": Misc > Send Intent
Edit the parameters (Usually fill in the Action and Extra is enough)


Ref: http://tasker.dinglisch.net/userguide/en/intents.html

P.S. Some famous companies even provide you the intent details on their developer website, such as Evernote.