Google Analytics does not capture any DOM events. Data gets into the "Events" report in the GA interface when a developer explicitly makes a function call like `ga('send', 'event', Category, Action, Label);`. Google Tag Manager can automatically add tracking to several types of DOM interactions. GTM captures events at the document.body, and you would generally need a work-around if something is screwing with event bubbling.

Nowadays, connecting Sessions and Users are handled by storing an ID in a first-party cookie named _ga. There is no such thing as an Event ID in Google Analytics, unless you implement one manually in a Custom Dimension. The Event report has Total Event and Unique Event metrics, and it's up to the user to choose whether they want to pay attention to deduplicated Events or not.

On the web, hits are not batched, they are handled as they come in (except for a throttling limit). The Android and iOS SDK batch hits by default.

On pageload, queued hits can be sent with the "beacon" protocol, which is a HTML standard that exists specifically to solve this exact problem. In browsers that do not use the beacon protocol, the hit is simply dropped.

There's no magic to tracking cookies, unless you don't care about regulation, why not use your own tracking cookies for this?

How old is "old", and what do you need it for? I've written code to parse the utmz cookie and separate code to recreate the whole organic/referral/utm attribution model in GA using first party log data. The only major drawback AFAIK is that adwords utm parameters are obfuscated and Google doesn't provide a way to resolve glclids to their campaign's utm params.