For the first two issues you mention:
1) Check the loadout screen, in case the tail gunner's ammo load has been set to empty. It's even possible to completely remove the gun. Why? Well, maybe so that in the future we can have variations of multi-role aircraft with a single flyable slot: change between a glass nose or a gun nose for Ju88s and mosquitoes, or remove guns to save weight if flying a high altitude reconnaissance mission, etc.
In any case, just make sure your gunner has both a gun and bullets to fire
2) There is indeed a mismatch between the fuel selector switch positions in the 110 cockpit, the info window messages (the "secondary controls" information which on default settings appears in blue text on the right-hand side of the screen) and the actual tanks. Tanks and fuel gauges correspond correctly to each other, but the info window information doesn't.
As a short term workaround, ignore the interface text and look at the actual cockpit control, it's a two-piece lever. It seems the forward part of the lever is for the forward tanks and the rearward part of it is for the rear tanks.
So, to select the front left tank, click on it or press the buttons you mapped to it until you see the rearward part of the lever centered and the forward part of the lever pointing left.
If you want to select the rear starboard tank, change it so that the front part of the lever is centered and the rear part points to the right.
I haven't flown the 110 much since the last patch so this is directly from memory and might be a bit inaccurate, but that's the general idea.
Hope it helps.