High kingship over the territory delineated in your first map in the TL?
On territories but as well their populations : we know that they were instances of peeoples essentially forming true federations (Remi and Suessiones, for instances) and other (like Menapii or Arecomici) splitting over the main people territory to have their own "city", with black jack and hookers.
What about the Upper Danube basin and Pannonia? Also would oppida still remain prominent there with a Germanic influx or just passage of time?
Frankly : it's anyone's guess and it depens on what happen on the Pontic Steppe and Balkans. I'd tend to think Raetia and Bohemia have fair chances to have a distinct Celtic background with a more or less important Germanic part, but that's me.
I'm wondering, how plausible is the idea that various rulers(Ariovistus for example) had more than one name or nickname for various linguistic group that said ruler dealt with? For example "Attila" is possibly a Gothic nickname.
Or it's, simply, their names. I agree that names aren't by themselves proof of cultural background (my own name have an Hebrew origin and I just don't think I've an Israeli background) but it certainly highlights some cultural presence (and for a king/leader, it's far than irrelevant). Regarding Huns, consiering that Attila, his predecessors (Bleda and Ruga) or his son Ellac were probably Germanic or Germanized names, it might not be an oversight of litterally every contemprary scholars but his actual name (or at the very least its usual name).
There's some circular reasoning thinking that, because he's an Hun, his real name must have been Hunnic (even if we couldn't recognize Hunnic anguage if he jumped all over us and trampled our head asking for tribute), and he must have been Hunnic and not Germanic because Attilla was a Hun.
The same goes for Arviosist and the whole of names rather well explainable trough Gaulish : that they appear regularily does points to a likely mixed cultural bag in the upper layers of these peoples at least.
Also prior to Rome's intervention, what territories did this intermixed region compromise? How far did Celtic influence go northwards and how far did Germanic?
It's really hard to say, because without written evidence we have to look at material culture : and then we're reminded that several Celtic features are often regional and their absence doesn't mean they were not related. No oppidae doesn't necessarily mean it wasn't Celt, just that it wasn't similar to this Celtic culture.
Conversely,
what does artifacts like this tell us? That Celtic features were present as far as Danemark? That Germans and Celts of Germania exchanged goods and practices?
While we can define the extent of Celtic culture trough Halstattian and Latenian material culture evidence, it's not this much decisive in making the difference between Celts and Germans (possibly more or less importantly Celtized, but also dynamic enough to advance south). Similarily, we're able to find the rough limits of Iron Age Scandinavian/Geranic culture, but there's a whole region in between that it's hard to attribute after the IInd century BCE.
So, its really only safe to depict Middle-Germany as a mixed area after the IInd century BCE, and before that possibly Celtic as long material culture is concerned.
A march isn't necessarily a military march, but also a buffer region, a border zone, etc. In this sense, a set of peoples influenced by their neighbours, not exactly part of their neighbour ensemble and essentially defined by being the "Last Chance Saloon" of local geopolitics.