So these rubber bladders don't seem to bring much to the party. True wet wings seem a far better solution.
It all depends... most small planes have bladders or built in tanks so they can put other stuff through the wing around the tanks, stuff like aileron push rods, pitot lines and electrical wiring, and they don't need that much fuel anyway, the fuel tanks in a typical Cessna 1xx wing don't take up that much room compared the size of the whole wing.
Most big planes are wet wing or a mix, because they want to carry as much fuel as possible, and the wings are designed with that in mind. Also they have room in front of the fwd spar or behind the center or aft spar to run wiring and hydraulics and air ducts and all that. I know on a 6 the reason for the bladders behind the center spar is because they run air ducts, fuel crossfeed lines and aileron control cables through there. Engine control cables, fire suppression lines and wiring are all run in the leading edge area in front of the fwd spar. There is nothing in the wet wing area between the fwd and center spar except boost pumps and capacitive fuel probes.
But these microorganisms and other nasties don't attack the aluminium-alloy structures?
This is really only a problem with planes that sit idle for long periods. Working planes which fly often burn through the fuel quick enough that microbials don't have a chance to grow. (also should be sumping tanks often to ensure there is no water in the bottom) Also there are fuel additives designed to prevent such growth. As for corrosion, they can be corrosive, but that is of secondary concern to them causing fuel starvation, and the aluminum would be alodined against corrosion.
Is the whole surface protected, anodized or coated with Proseal or is it only on areas where a leak might be anticipated? Are magnesium-alloys a no-no for raw fuel?
Proseal just the rivet lines and fittings. Top coat everywhere (the top coat is only as thick as paint and brushes on like it) The aluminum is also all alodined. There are many flavors of proseal too, you used different stuff for avgas and jet A, different consistencies, set times etc.. I don't believe that fuel is bad for magnesium, but magnesium alloys are not used for skin, there are often many magnesium parts on aircraft, but they are usually fittings and brackets and wheels, big thick pieces. Skins on metal airplanes are mostly aluminum, with stainless steel and titanium also used in some places as needed.
Now, in modern planes with composite wings... I admit I have no idea how the wet wings are built or sealed.
I saw a video about what you were talking about Vraciu. Saw a big dude wriggle into the oval access hatch of a jet airliner. He had to put his hands at his sides and go head-first. Must have been a miniature gynochologist in a former life 
It is usually the small guys who get drafted into "tank diving". You definitely can't be claustrophobic and do it. I'm big enough that I can only fit my head and one shoulder into a DC-6 oval panel, it's the skinny guys who draw that job.
