Remember that if you don't fly to EBBR by yourself, you're looking at another hour of travel time (even if you keep a car at the airport you fly to).
Depends on where you're going. If it's near the airport, EBBR is easier by an hour. For central Brussels, the difference between EBBR (on the bus or taxi) and the regional airports (by train) is much less.
2. Same thing as (1), but in a G-registered Cessna 206. The operator is an LLC based in Lewisham, which also owns the aircraft.
I know nothing about FAA rules (though I believe N-reg cost-sharing in the UK is not permitted without a DfT permission -- Art 223). The G-reg scenario you describe seems reasonable for 4 under current UK legislation, but as 500 above says, there's a limit of 4 people for cost sharing. No one really knows what the "future rules" are in this area, or rather, no one knows how they will be interpreted.
I agree that if you have a small party of people, it may be cheaper to fly yourself. But I wouldn't bet on it.