The first bug is when I fish on the river or the sea... the character looks and throws the bait on land instead of the river/sea, although I fish without a problem.. But the bug that breaks my game is when I deliver the health potion to Crotho.. she says her lines and when the quest is complete the game freezes.. can't save, can't move, can't open inventory... only force close it..

Happened to me too. According to the patch notes on steam the issue have been fixed in 1.201, but the GOG version seems to still be on 1.200. Giving the cauldron instead worked for me to progress the quest.

Yes, I had enough silver to get the cauldron so I just sold huge amounts of coal to the blacksmith to get to Rank 2 vendor items to buy it and test it. It worked and I progressed smoothly. Haven't seen any other bug for now except the reverse fishing one at the waterfall and at the sea.

And yet, the graveyard quality is not calculated correctly. Is that not a rather serious bug? I have a body quality of -1, +2 for a stone headstone and +2 for a stone fence, yet the grave has a final quality score of +1. Why? It had had wooden items before, but the quality has not updated to reflect the newly crafted stone items I installed. I really cannot remember that Steam had that bug.

It's not a bug, its a game rule :)
If you check your tomb window, you'll see a specific number of skulls.
That number depends on the quality of the corpse you buried.
For example 2 skulls for a -1 corpse (not sure if the ratio is correct but its only an example)
If your decoration exceed 2 skulls, it wont be taken into account ^^
Better take away that headstone and use it with another tomb ;)

Which means you have to bury corpses in a pretty "good shape" xD to have several skulls unlocked for your decoration ^^