It's funny, because I think it makes perfect sense for all those reasons. But even taking a step back - the explanation was that he was going to "calculate" where the ball would land similar to how a computer would. There are computers which can do this, but they certainly don't give you THE number, they can indicate an area of the wheel but even then it's not 100%. You're happy that this is what he was doing? If that's the case, he could do it over and over. He could have made a follow-up, just for his website, where he showed us him doing this (instead of posting that Stewie impression)
The casino footage was definitely fake. Most tellingly, the audio didn't match with the visual when Derren approached the table; the croupier called a different number to what the screen showed the ball was in. But anyway, he was also already filming with a couple of cameras that he'd already shown us to be the size of a healthy dildo, despite insisting that you couldn't get away with any kind of electical equipment (the computer to do the job would be a fraction of the size).
And besides - getting one out is still impressive. So even if it was a genuine attempt and you (and others) deem it an embarrassing failure, I wouldn't. But that's beside the point, since it wasn't. But that's why the fake footage was necessary, to make it a near miss. (And also, of course, to avoid the risk of filming live in a real casino, which he possibly/probably wouldn't have got away with.)
I also disagree that it wasn't funny the way it was done. If I take it completely at face value (which I'm still not sure is correct btw) the joke was wholly on Ben, since he would have definitely been expecting the payout. (Of course, the only way he could have got the payout is if it came out of Derren's own pocket)