- When a user registration is complete?

There's a trigger for registrations so yes I suppose.

- When a user places a review (Jlex Review component)?
- Invites a contact?

CB Auto Actions can act on Joomla events using its Joomla system plugin so as long as your 3rd party extension has a Joomla event for that then yes.

- Share on Facebook/Twitter/WhatsApp?

No, that'd require a Joomla extension to utilize those 3rd party APIs to scan for a user actually doing that. If you have such an extension AND it fires a Joomla event then yes, but I'm guessing it's unlikely such an extension exists.

And than the most important feature: Can a user exchange his or her points for discount on a product/service via an external component?
For example: 100 user/reward points can be exchanged for 10% discount on a cup of coffee (voucher).

No idea. Would depend entirely on how you store points and how your external component utilizes them. For example if you use a Points field in CB you could use that in CBSubs with CBSubs Promotions to add discounts to subscriptions.

The invites_onAfterAcceptInvite trigger is fired when an invite has been accepted. For that trigger var1 is the invite object and var2 is the user accepting the invite. So for your auto action you probably need something like the below.

Triggers: invites_onAfterAcceptInvite
User: Manually
User Variable: Variable 1

This should ensure your action will apply to the user who sent to invite. If not give the below a try.

Triggers: invites_onAfterAcceptInvite
User: Specific
User IDs: [var1_user_id]