1) Add all new users directly to the free plan.

Using CB Auto Actions act on the after registration trigger in a CB Paid Subscriptions action and subscribe them to your free plan. Alternatively make all your plans child plans of your free plan as the parent is always required to subscribe to its child plans.

2) If their current subscription ends they automatically get assigned to the free plan.

This can be done with the above suggestion of making them child plans of the free plan as they'll never lose the free plan. You can act on the below trigger usages to subscribe them to the free plan when another plan expires if you don't want them to have both plans at once.

Plan Active
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditional 1: [var3] Equal To PLAN_ID_HERE
Conditional 2: [var2] Equal To A

Plan Expired
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditional 1: [var3] Equal To PLAN_ID_HERE
Conditional 2: [var2] Equal To X

3) All Free plan subscribers should get a fortnightly email automatically encouraging them to upgrade to other plans with more features.

See your documentation in regards to CBSubs Mailer as it can be used to send reminders, etc..

var3 for that trigger is the CBSubs Plan ID (far right column in CBSubs > Plans). var2 is the subscription state. A is active and X is deactivated.

1) made all plans child plans of the free plan.

You don't need 1 or 2 if you've done this. Since all your paid plans are now child plans of the free plan they can't subscribe to any of them without first subscribing to the free plan. Making the free plan exclusive (mandatory) will require it to be selected during registration.

2) Created an auto action to subscribe the new users to the free plan.

Are you testing frontend or backend? Your current usage is only going to work in frontend. If your plan is free lifetime you also need to ensure free lifetime plans are configured to give subscriptions within CBSubs > Settings > Global.