I've heard of bettas swimming up into filters so it is possible. All you can do as a fish owner is baffle the filter so it becomes harder for the fish to get up there. If it does happen you've already done everything you could to prevent curiousity from killing the fish. (I know, I know. Not punny XD)
If you really are that worried about your fish getting into the baffle, then you can baffle with aquarium sponge. However, I do have more current in my tank than is suggested. I guess you could stick aquarium sponge into the baffle. Water goes right through it and it would be a physical barricade to prevent your fish from going into the baffle. But if your fish is very determined (like Snowy), you will see the aqurium sponge floating in a corner of your tank and you betty looking at you with a halo over his head. I'll answer your second question just to prove how suicidal Bettas can be. There is a point where you can only shrug and say, "I tried to warn you but you just had to..." It also explains why I had to get creative with blocking off the filter.
I know Snowy would be dead already with a conventional baffle if he can knock the sponge out of place. I had to get a bigger aquarium sponge (for 70gallon tanks) so the tank cover holds down the sponge to keep the baracade in place. I have no idea why Snowy likes playing around the sponge so much. He stopped a bit now because the sponge doesn't move, but I thought I would come home to a disaster for about a week.
Luigi's insanity is a bit tamer. He likes to swim near the filter intake, float, and slowly get pulled up againts the filter. He swims away from it when he gets bored so I doubt it's due to excessively strong a suction. I think Luigi treats it like an amusement park ride or something. That section is also baffled with aquarium sponge, so his fins shouldn't get sucked into the vent and water can still flow through. But I wouldn't be surprised if I can home to find Luigi swimming around in circles because he only has one fin. He doesn't even bother flaring when I stick my face up against the tank to look at his fins to make sure they are all healthy.
So...yeah....I could write a book about fish that think up new ways to give their owners heart attacks. XD