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.

1) Using live report, you should be able to see the volumes flowing into the journeys / campaignshttps://experienceleague.adobe.com/en/docs/journey-optimizer/using/reporting/live-report/live-reporthttps://experienceleague.adobe.com/en/docs/journey-optimizer/using/reporting/live-report/campaign-live-...

Kindly log a support ticket for this in case you don't see any errors in journey step events dataset 

Javascript is not supported currently and instead, personalization in Journey Optimizer is based on the templating syntax called Handlebars.  Referencehttps://experienceleague.adobe.com/en/docs/journey-optimizer/using/content-management/personalization/personalization-syntax  You can take a look at ...

No, it is a single field that can be updated in a given update activity.https://experienceleague.adobe.com/en/docs/journey-optimizer/using/orchestrate-journeys/about-journey-building/update-profiles

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