The reason that you carbon rig worked well was that it was engineered and built correctly. A lot of so-called composite masts tumble down due to all sort of reasons as do alloy masts. That been poor engineering, poor build quality and sailing accidences.
You stated that your alloy mast
supplier supplied upgrades to your spreaders (The angle of them) this was done to either support the mast
Fore/Aft or athwart ships depending on the new angle supplied less angle for athwart ships and more angle fore and aft.
If the rig was a double diamond with no forward jumpers on the mast (a single
spreader facing forward at both top and bottom diamonds with wires running over both) and the mast was going out of column then the moment was never an enough in the mast for this configuration.
Reef heights have to be taken into calculation as well. When deep reefed the load on the mast at the headboard is very high and this is tring to invert the mast as the loads are transferred down the leech of the sail to the main sheet
There are several rig configurations available to multihull
owners when purchasing
If people wish I will go through them and advise the pros and cons for each design, the safest and most fail-safe.
The best form of protection against corrosion
is an anodized mast or if you have the money
a professionally built carbon mast.