Some answers:
1) Are speaker impedance curves (SIC) loaded by default with an amp when its associated Cab block is linked (gold link icon) ?
-> Yes
2) Are SIC loaded by default with an amp when an IR Cab block is inserted afterwards ? If yes, which SIC (i.e., from which cab)?
-> Yes, the default cab.
3) What is the difference between a linked and unlinked IR Cab block (you can have both option) ?
-> My guess: with link icon = default SIC loaded, without link icon = no SIC ?
4) Are SIC loaded by default with an amp when no cab block is inserted afterwards (when using with a real power amp and cab)?
-> My guess: No ?