The quantities in the kits have been sized to ensure the coverage is that figure, however 0.2sqm is not a lot extra. Due to pack sizes, you have enough gelcoat and coupling coat to do it no problem, but you would need some more Tooling resin and CSM to be on the safe side.
If you are using extra wood formers after the main mould making to add stiffness, then no need for extra thickness. You can add the extra support using the wood.
I don't see why a silicone former wouldn't work. You would get resin flash all around it as its unlikely to be perfectly sealed, but it would do the job of preventing the resin filling the voids. Just take care to ensure its presence doesn't lead to any bridging of the fabric or bagging stack.