Part 1 (dative vs accusative)
The preposition »über« is a Wechselpräposition (alternating preposition; change preposition). Such prepositions can have a dative object or also an accusative object:
accusative
Die Drohne fliegt über den Fluss.
The drone flies across the river.
You use accusative to describe a movement from one place to another. Before the action began, the drone was on one side of the river, then it flew across it and ends on the other side of the river. It is therefore in a different place at the end than at the beginning.
dative
Die Drohne fliegt über dem Fluss.
The drone flies above the river.
The drone is either hovering in the air, somewhere above the river, or it follows the course of the river. While it's flying, it doesn't leave the realm of the river. You use dative to describe an action that happens stationary (even if it's a movement, as long as this movement happens entirely inside a given region). The drone is above the river all the time: at the beginning and at the end and at all times in between.
This difference between a stationary action and a movement from A to B is relatively easy to understand when it's about local aspects (places and directions). But this distinction between stationary actions and movements also works for temporal prepositions:
Examples:
(I need to use a preposition other than über here, and I'll explain why later.)
The preposition »über« can be used also as a temporal preposition, but only in combination with periods of time, which counts as a temporal movement. So, when you use »über« in a temporal manner, you can't combine it with dative. Only accusative is possible:
Tante Helga bleibt über die Feiertage bei uns.
Aunt Helga is staying with us over the holidays.
But when a preposition is not used local or temporal, it makes no sense to talk about circumstances or reasons being stationary or moving. And most of the Wechselpräpositionen even can't be used other than local or temporal, but über can.
Most grammar books usually list only four types of usage for prepositions: Local and temporal which we both already know, but also modal (describing how something happens: »Er isst mit großem Appetit.«) and causal (describing why something happens: »Sie kam wegen der Nachspeise.«) But this list ist not complete. The usage of »über« in »Markus redet über ihn« belongs to none of these four types. Some authors introduce a relational use of prepositions that would fit your sentence well, but more commonly, uses that do not fit the four common types are grouped into a fifth type called a "neutral preposition."
So, the question of classifying the kind of usage in »über jemanden reden« is tricky, but the answer is definitely neither "local" nor "temporal" and for that reason you don't need to care about the fact, that »über« is a Wechselpräposition. If it's neither local nor temporal, always use accusative case.
Part 2 (can »über jemanden reden« also mean »to speak well about someone«?)
Under certain circumstances, it can be used that way, but usually not.
Generally spoken »über jemanden reden« means that there are rumors about someone. Rumors can have a positive content (»Some people say, she could be the winner of the next Nobel price«) but rumors usually have less nice contents (»I saw her together with Georg conspicuously often. I think she is cheating on her husband.«) And this is the reason, why »über jemanden reden« usually has a negative connotation.
