Correct, it is just one custom action. What is the error you are running into ?

You can use it on the following lines, specifying the type of authorization as "Custom Authorization" This will incorporate the access token in the outbound api call via custom actions https://experienceleague.adobe.com/en/docs/journey-optimizer/using/configuration/configure-journeys/data-source-jou...

Could you share the requirement as to why response headers are needed ?

Can you share the use case details where you would need this information ?

Not from a package, but you can explore leveraging https://developer.adobe.com/journey-optimizer-apis/references/content/#tag/Content-template-API for importing the HTML content.

When you open the tracked URL's in Email designer, you can configure the tracking type as External Opt-out / one click opt out and this config. should control how _experience.customerJourneyManagement.messageInteraction.interactionType is populated.   

You can try this{% let idx = 0 %}{{#each profile._company.LoyaltyCards as |p|}}{%#if startsWith(p,"2616") and idx < 1 %}{{p}}{% let idx = idx + 1%}{%/if%}{{/each}}

This is a guardrail called out in https://experienceleague.adobe.com/en/docs/journey-optimizer/using/get-started/guardrails You can potentially explore leveraging a middleware/custom solution for this. 

You can have a helper on the following lines -  {{#each profile._company.LoyaltyCards.LoyaltyCardNumber as |p|}}{%#if startsWith(p,"2616") %}{{p}}{%/if%}{{/each}} https://experienceleague.adobe.com/en/docs/journey-optimizer/using/content-management/personalization/functions/string#startswithhttps://...

Condition activity evaluates to a boolean value meaning if the criteria satisfies, happy path is taken and if it doesn't, profile goes down the other's path. You can adapt it based on the below example Example - #{ExperiencePlatform.ProfileFieldGroup.profile.homeAddress.city} == "San Jose"