Some airplanes you have a separate position for the gunner or bombardier.
If you move to the gunner seat in the Aviatik, does the plane keep altitude and or course.
And if so, does the plane try and maneuver away (or go after) enemy planes in a dogfight if you occupy the gunner seat and the AI is controlling the pilot?
If not, and you have to remain the cockpit seat, then how do you have the gunner fire the gun in defense or offence scenarios.

the pilot will fly the plane for you.

2-seater: your toast if a fight happens. The pilot will fly straight and level when ever he can, and you'll find your tail getting in the way of clear shots.

The "Roland" pilot will do his best to stunt with opposition, but the thing is a brick and will eventually be flying at ground level with the enemy on your tail again.
The Roland pilot will almost always engage, even giving up thousands of feet to do so even if you didn't want him to do so.

Many have tried this career, but no one actually lasted very long.

No, not by default. You must turn on AUTO PILOT ctrl+shift+A if you want him to fly for you, and if you switch back turn it off if you want to fly.

If you turn AP on at the start you can sit the entire mission as a gunner/observer, as the AP will take off, fly to the mission and fly back.
Sometimes when you are trying to line up a shot turn off AP so the craft will be not controlled briefly and you may get your shot off better then turn AP back on.

first order of business would be to fly each two-seater in free flight in the observer seat. then sweep the gun through it's entire range of motion (up and down, side to side). get this burned into your memory. your AI observer can only open fire if he can actually point his gun at the enemy. if you're in a fight, your job as the pilot is to either run away and/or fly in such a way as to allow your observer to fight back. I find that it is generally best to fly as the pilot. there are a handful of machines where you could plausibly fly as just an observer.... but that's probably only the FE2b and the Caudron G.IV. that's because they both fairly early war planes that have a bow and stern machine gun. (you could probably do this with the Bristol Fighter F2b as well)

Polovski is right. you activate the full auto-pilot with CTRL+Shift+A in order to jump into the observer seat and fight off pursuers. the default is that the aircraft essentially goes 'dead stick' once you leave the cockpit. with some machines, like the BE2c this is perfectly acceptable - as it was a very stable flying machine. in other machines, like the Sopwith Strutter or the Caudron, it's likely to start listing over to one side.

how often you do switch from full AP to manual control will vary from one plane to another. it also depends on how aggressive your observer is. if you have an experienced AI observer you might be better off just flying the machine and letting your AI observer shoot up the enemy.

that being said, even pretty good observers will open fire at ranges that are simply too far. so it's not uncommon for me to set up a pursuit path and fly auto-level if I know I have a faster two-seater. by flying auto-level and taking up the observer station I can frequently sneak below and behind enemy aircraft and shoot them up from the observer's station in the nose of my Caudron G.IV. it requires a lot of careful planning and patience, though. and you are not at all guaranteed to shoot them down. even if you DO shoot them down you will stand a pretty good chance of not getting the claim confirmed if you lose wingmen in the process. although I haven't become a confirmed ace in the Caudron yet with any one particular pilot I've managed to shoot down several dozen German aircraft using this type.

I think an 'observer' career is possible... but I agree that it can't really be done with the Roland C.II - that's a plane where you want to fly it as a pilot far more often than not. besides, it has a fixed forward firing machine gun.... so as long as you manage your speed and altitude properly and pick your battles very carefully.... you can treat it like a fighter.

Always thought a career as a Fee gunner (or Brisfit) would be very cool.

I'm flying a Roland CII at the moment and I'm 50 missions in so far. It's definitely not a plane for stunting and twist and turn fighting but it's quick enough (by mid 1916 standards anyway) so you can catch a DH2 up from behind for instance. I like it because you have the best of both worlds, bombing and recon missions and the opportunity to fly as a fighter as well. With the added bonus of having an armed observer....which is very useful in shaking off fighters on your tail or dealing with tricky two seaters....Morane Parasols for instance.

It's about picking the right fights for the aircraft you're flying.