It's currently not feasible. Supported ways of consuming assets are documented in this link https://experienceleague.adobe.com/en/docs/journey-optimizer/using/content-management/assets-images/assets#assets-prerequisites 

I am assuming you are relying on productListItems object array to know the number of items in the cart. If the productListItems count of checkout event is equal to count of productListItems count of the product removals since the checkout was started in the last x hours, that should give an indicati...

You can use collection management functions https://experienceleague.adobe.com/en/docs/journey-optimizer/using/orchestrate-journeys/building-advanced-conditions-journeys/syntax/collection-management-functions to perform this check to see if there is at least one check out event and no product remova...

Take a look at this note on how profile estimates are generated. https://experienceleague.adobe.com/en/docs/experience-platform/segmentation/api/previews-and-estimates#how-estimates-are-generated  You can check for the segment membership of this profile under profile attributes or segment membership...

You can use it on the following lines {% let d = formatDate(toDateTime(<balance_date>, "Etc/GMT"),"MM/dd/YYYY") %} {{d}}https://experienceleague.adobe.com/en/docs/journey-optimizer/using/content-management/personalization/functions/dates#format-date   

Looks like you are using custom actions which is actually performing the send outs. Atleast from the journey side, you are handing off the payload to a REST API end point. You will need to check if the service behind the API end point has the recall facility.

You can use "Exclude test events" option in the reports https://experienceleague.adobe.com/en/docs/journey-optimizer/using/reporting/live-report/live-report 

You can explore the feasibility of directly checking this in the segmentation criteria, itself. This segment can be used in a batch journey (say, that runs daily) Target users who received a welcome email 30 days before today and there is no app login event detected. 

can you do a profile lookup and see if the identityMap has the namespace populated that you are looking for ?

For unitary journeys (event based and audience qualification), emails will be sent to addresses in seedlist at the time of first send only.