Here is the reference link on how to add links in the email content https://experienceleague.adobe.com/en/docs/journey-optimizer/using/channels/email/design-email/add-content/message-tracking 

I believe the response payload is configured in the custom action response. If so, can you try accessing the response attributes in a condition and see if you notice it in the logs ?

Take a look at this related thread https://experienceleaguecommunities.adobe.com/t5/adobe-experience-platform/update-profile-to-use-in-other-journeys/m-p/685770 

Can you share the journey flow ?

Try validating the namespace and the event definition with the actual event data that is coming in, to see if that meets the criteria. Also, by default email execution address is set to personalEmail.address  - you may want to overwrite it (if this is not being populated) with the attribute that con...

No, this isn't feasible currently

Not exactly capping but you can control the reading rate in case of a batch journey using a combination of waits and percentages https://experienceleague.adobe.com/en/docs/journey-optimizer/using/orchestrate-journeys/journey-use-cases/technical-use-cases/limit-throughput and explore profile cap opti...

Couple of options - If you have a REST API service which can return a coupon code for every request made, the same can be configured in custom actions and custom action response can then be used in emails for personalization https://experienceleague.adobe.com/en/docs/journey-optimizer/using/configur...

You can prepare a json file populating the subscription map for each profile and load it against the dataset containing consent and preferences field group attributes. loyalty-offers and newsletters are subscription list in this example.Reference - https://experienceleague.adobe.com/en/docs/experien...

Expression Fragments can be used in the personalization editor (for email and sms channels) and as part of the recent enhancements, are available for In app and web channels too.