Kindly check if discover.newProduct value is added to the event type attribute as enum / suggested values (Reference - https://experienceleague.adobe.com/en/docs/experience-platform/xdm/ui/fields/enum)  If so, try creating a new event and see if it shows up. As an alternative, you can use advanced m...

Kindly log a support ticket for this.

You can create a batch segment leveraging the attributes from message feedback event schema and accordingly suppress it, within the journey. Assumption is, profiles had the message node processed in journey. However, you need to wait for the daily batch segmentation job to be run before you can use ...

It will be on the following lines if (a=="1") then ("XYZ") else ( if (a=="2") then ("XYZ") else ( if (a=="3") then ("XYZ") else ("No Segment") ) )

Can you share the screenshot of what error you are running into ?

you will need to access it via the message node or the simulate content option. Referenceshttps://experienceleague.adobe.com/en/docs/journey-optimizer/using/landing-pages/create-lp#test-landing-pagehttps://experienceleague.adobe.com/en/docs/journey-optimizer/using/landing-pages/get-started-lp 

This API is for listing the endpoint capping configurations and not the journey details.

You can explore the API route and in the response, you can see the rules configured in the audiences.  Reference - https://experienceleague.adobe.com/en/docs/experience-platform/segmentation/api/audiences#:~:text=An%20audience%20is%20a%20collection,Platform%20or%20from%20external%20sources.  

Not in the current state,  kindly log an enhancement for this.

No, other than the technical properties in the journey canvas, it is currently not stored in the system datasets.