Earlier quoted context omitted.
That's total, not per block. Per block, fees average 0.9btc iirc. Transaction reward is 12.5 btc 0.9 is not insignificant, but compared with block reward, the true (current) incentive providing block security is not in question, and that is what was being discussed.
Per block or per day is the same thing. Also better to look at it on a day level since transactions+fees vary throughout the day
Because we are talking about block security. Security is not on a per day basis, its on a block basis.
people don't make race attacks on a "day" they make a race attack on a block or series of blocks.