Sealing gaps only reduces thechance of flutter a little... the flex in the hinge line will be the factor that determines if flutter is allowed to initiate.

The sealing will reduce turbulence over the control surface slightly... and may delay flutter onset, but that's all, it won't prevent it. Once flutter starts... you have to slwo down enough to stop it. If the hinge line is too flexible, then even with the sealing, you might stall before the flutter stops.

I've had to land a plane that had an aileron fluttering at anything greater than about 1 mph above stall speed... Its not fun. (something about losing 1/4 of one wing and snapping that aileron's clevice in a mid-air messed up my model's aerodynamics...) i would imagine trying to do that with the elevator wanting to flutter at anything above stall would be a worse nightmare.

Better safe than sorry. Rehinge it.

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If a beginner brought that to our field... and I had my stuff with me (usually do when I know beginners will be comming...) I would personally cut the hinges... and rehinge it correctly. I've done it before...