2/ In the old wallet UI, I don't find how to import a wallet.dat file so that I can at least find again my old holding addresses.
Answer to my own questions:
1/ node list here: https://chainz.cryptoid.info/mue-old/#!network
2/ just copying the wallet.dat in the working folder of the client will do it
Edit:
I found the cause of the mess. The old and new wallets are actually conflicting. Their "work" directory (The directory were the blockchain data and wallet data, etc. is stored) is set in the windows registry here:
Computer\HKEY_USERS\
in strDataDir
Both the old and new wallet are looking up the reg path stored here. So if you run the new wallet on a PC with the old wallet installed and running or vice verca they will overwrite eachother's data. !!Warning!! This is including your wallet data, so if you don't back it up, you have all chances to loose coins by running old an new wallets on the same PC.