I'm currently at the first immaculate trial where you have to place objects on the four switches. I figured three of them out so far just by dumb luck and trial&error, but I wonder if there's actually any correct way to deduce a solution? Are there any hints I can find beside the statue's vague talk about 'the weight of knowledge'?

There's really nothing, it's a trial and error puzzle. The one that stumped me the longest was the one to the right from the first pair of switches, I eventually cleared it with two candles.

I read somewhere her about 1, 2.5, 5 and 7.5 kg so I use:
- hammer
- bucket with water, maybe also some food on top
- Vase from there
- grate with golden plates frome homestad and some food

(must reload to search for this things outside)

All is correct, except the 2nd one. I just used a 2lb hammer and it worked instead of a 2.5lb object or objects.

A single item of candles is enough for the smallest one, a barrel is specifically labelled 'heavy barrel', this will get the largest, a bucket of water and pot will get the other two. No need to mess around with item combinations (nice job with the improvisation though).

Thanks for this post. This is quite possibly the most idiotic puzzle I've seen in a game.

For anyone stuck, to summarize the weights are: 1, 2, 5, 7.5 from smallest to largest button.

The clues are there by the unique descriptors of two of the required items, the unique name for one of the items, and then just a nearby item.

Top: Book of Immaculates II (near the fallen bookshelf in the area)
Right: Bucket of Water (need to press a switch to open a hidden passage to find this)
Left: Heavy Barrel (down the staircase passageway just next to it)
Bottom: Broken Vase (from nearby)

While there are a lot of puzzles I dislike and some i have asked for help or googl'ed; this specific puzzle I did on my own and it was quite easy so I don't think i would call it idiotic. I did the first three easy; I was really worried that the last one was going to take a while to guess but then I found an item in the room that was the exact weight I needed and it was obvious when I found the item as to the usage. The two puzzles I found really painful were dodging the death knight (who have very long vision and run very fast) and the mirror (one of which is hidden); oh yea and the 4th button in the chapel (found 3 but not the 4th).

I've destroyed the heavy barrel which is for the large switch plate. Can I use a normal barrel which is 5 plus some other items?

yes