The battery (Gill 241) has a kind of vent
Yes, interesting. I used to have the 242 which didn't have a single vent. You've got to make sure it doesn't get blocked.
Finally, the charging current with engine running is ca. 6A. Charger gives only about 3A.
The first should be a lot more than 6A. If my battery is a bit low, I see about 30A going into it, for some minutes. If you see only 6A, ever, there is something wrong with your alternator, IMHO. The charger will supply whatever it is designed to supply.
Can I use the supplied lead with rings permanently mounted on the battery? Is it legal?
This one has been done to death on American forums. The standard answer is that it is at least a Minor mod (a logbook entry) and possibly worse. The practical answer is that you must have a
fuse in that lead, in the non-airframe side (usually the positive one) and the fuse needs to be as close to the battery as possible, in case the lead shorts to the airframe further down.
Lots of planes have direct battery connections (e.g. mine has a wire for the yoke clock and the courtesy light; the stupid light is a great way to flatten the battery) and a lot of people have a mod done whereby one of their radios is connected directly to the battery, but these always use an inline fuse close to the battery.
There is a serious fire hazard in this because a short circuit, without a fuse, will instantly set the wire on fire.
would be a charger for "aviation use" it would be "more legal" (insurance issue)
I doubt it... all this stuff is "portable". I know some hangar owners insist on an electrical tesf for all equipment used in the hangar.