When I looked at tornado structures (or building one) it was mentioned that if the door opened outward, there was a possibility of it being blocked by wreckage that would stop it from swinging open.

OTOH, if it swings inward, the only thing that keeps it from opening is the latch itself, not the whole door frame.

You pick your choices and take your chances I guess! If I was building one, I don't know what route I'd take.

Are the blocks filled with concrete and reinforced with rebar?

The Sporting Clays range that I frequent built a vault like you did, although they have an actual vault type door on it. They filled the blocks with cement and rebar, and also poured a 6" cement ceiling on it, supported by steel beams on the inside.

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