Short Question: Is :db/id -1 guaranteed to be INCREASING ? Long Question: I want to sort a list of entities by TIME-WHEN-INSERTED-INTO-DB; do I need to create a separate field, or can I Just hijack :db/id, assuming they were all inserted with -1 ?

You cannot sort entities by the time they were inserted. You can use the datoms api, like (d/datoms :eavt (:db/id entity)) and sort-by :tx to latest the datom of an entity

I need to ponder on this, clearly I'm misunderstanding something.

(defn- next-eid [db]
  (inc (:max-eid db)))

is :maz-eid something datascript provides, or am I injecting a new key in it myself?

it is internal in ds

https://github.com/tonsky/datascript/blob/master/src/datascript/db.cljc#L788