Here my experience with the mini, related to your questions:
1) Can it stay powered on 24h/24h without the risk of hang? I'm using amule on another machine and i want to use the ls mini as the storage (so the lsmini disks will work 24h/24h!, low transfer rate, but 24h/24h). Does this device usually hang?
I never had hang problems with my mini. (since Oct. 2010)
2) Is the lsmini very silent? (ok...disks do noise...but i think you cannot hear the noise if you're in another room in the night..or am i wrong?).
Yes, it is really silent. There is no fan or other "noise factor" beside the HDD's.
3) Not very important, but is it possible to install lenny or squeeze? (cannot find a REAL answer about this...)
4) Not very very important, is there a way of running torrent and amule on the same device at the same time?
Fist: Amule or torrent, are you serious?
Regarding your question: Torrent client runs with stock firmware (uTorrent embedded)
I don't know if amule runs ... but it should with linux on the mini ...
With an opened stock firmware running (http://buffalo.nas-central.org/wiki/Open_Stock_Firmware_LS-WXL
) you are able to use IPKG and install some other packages (services, programs)