Why is it that, both my 907 and 750 Paso have a (previous owner) history of explosions of the battery? Due to battery acid, the paint of frame and aluminium swing-arm are not that nice anymore...

I noticed that the battery is kept in place by rubber knobs and a sponge...
My '94 SuperSport had rubber fasteners over it which secured the battery tightly.

What causes those acid attacks?
What can I do to prevent it from happening again?

The acid attackes are caused by the battery overcharging and boiling the acid out. The root cause of the problem is the crappy electrics. With the voltage losses in the Paso system, the regulator "thinks" that the battery is low and charges the crap out of it. Some cleaning and rewiring will help alleviate the problem.

Maybe a nice Shell sticker to cover it up?? The anodize is impossible to replicate so it always looks damaged.

the problem with my swingarm is that there are miniscule putholes in the surface of the aluminium...
I guess those were camouflaged by the anodization of the swingarm, or the battery acid did the metal attack...
To smooth the surface I should sandpaper at least half a millimetre off......

solutions:
1. anodize the total swingarm (don't know if that is possible or easy to do with "old" metals (greasy, contaminated surface))
2. sandpaper/-blast the thing and spraypaint it (cheap)
3. sandblast and powdercoat the swingarm (stronger than paint)
4. look for another swingarm...

I think I am going for #3.

do not sandplast aluminium parts.
There`re less aggressive methods like ie glass pearl blasting (don`t know the correct english term)

furthermore note that powder coating is indeed a very resistant surface but if you need to retouch due to some scratches, things will become more difficult because there`s no good way to spot repair a scratched powder coat.

last comment. Bikeshops sell universal battery acid overflow cannisters which is nothing else than a plastic tube with one in and out tube on top of which one is connected to the battery. I had one of those and it only requires to take off the rear side fairing once in a while to check if this thing needed to be emptied. I had it mounted on the side of the battery compartment.
(after that I went for a no-maintainence sealed battery)